J. Electrochem. Soc. 2017 volume 164, issue 4, A655-A665, Feb 2, 2017
Many literature reports show that layered Li-Ni-Mn-Co oxides (NMC) have a surface reconstruction ... more Many literature reports show that layered Li-Ni-Mn-Co oxides (NMC) have a surface reconstruction to a rocksalt (Fm3m) structure which is claimed to be responsible for the increase in cell impedance during high voltage cycling. It is important to determine if appropriate electrolyte additives can suppress the surface reconstructions of NMC materials. LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811)/Graphite pouch cells with different electrolyte additives and different upper cutoff potentials were charge-discharge cycled and the electrodes were recovered for z-contrast scanning transmission electron microscope (STEM) studies. It was found that there was no significant surface layer growth for cells cycled between 2.8 and 4.1 V. For cells with an upper cutoff voltage of 4.3 V, the electrodes from cells with control electrolyte (no additives) showed the thickest surface layer. The electrolyte additives vinylene carbonate (VC) and prop-1-ene-1,3-sultone (PES) were found to suppress the growth of the surface layer. However, cells with PES showed a more rapid capacity fade than control cells or cells with 2% VC showing that, at least for NMC811/graphite cells with PES or VC additives, failure cannot only be solely ascribed to a growing rocksalt surface layer. Other processes, for example associated with electrolyte oxidation, are believed to be responsible for failure. High energy density lithium-ion batteries that are cheaper, safer and with longer lifetimes need to be developed in order to meet the increasing demand for applications such as electric vehicles. LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) can deliver a high capacity of ∼200 mAh/g with an average discharge voltage of ∼3.8 V (vs. Li+/Li), making it a promising positive electrode material for high energy density lithium-ion batteries. 1 However, electrochemical tests of NMC811 from half cells and full cells show poor cycling performance when charged to voltages above 4.2 V. 2 In-situ and ex-situ X-ray diffraction showed that there are no significant irreversible structural changes in the bulk of the material during charge-discharge cycling. Instead, the parasitic reactions between the electrolyte and the surface of the positive electrode particles at high voltages were suggested to be the cause of the failure of cells cycled above 4.2 V. 2 Layered NMC materials have a hexagonal layered structure (α-NaFeO 2-type structure described in the R ¯ m space group), where Li and transition metal atoms form alternating layers between oxygen layers and Li atoms have a 2-D diffusion path. 3–5 Recently, Lin et al. 6 showed that the surface of LiNi 0.42 Mn 0.42 Co 0.16 O 2 (NMC442) went through a structural reconstruction from layered (R ¯ m) to rocksalt (Fm3m). In that transition, transition metal ions migrated to the lithium layers with a possible loss of Li and O from the surface of the structure. This was cited as one of the causes of a significant increase in cell impedance under high voltage cycling conditions. This surface reconstruction phenomenon was also observed in many other reports about NMC and Li[Ni 0.80 Co 0.15 Al 0.05 ]O 2 positive electrode materials where the surface reconstruction was ascribed to be a result of interactions between the positive electrode surface and the electrolyte. 6–22
The long-term cycling behavior of 24 promising electrolyte blends were systematically studied in ... more The long-term cycling behavior of 24 promising electrolyte blends were systematically studied in LaPO 4-coated Li(Ni 0.4 Mn 0.4 Co 0.2)O 2 /graphite pouch type Li-ion cells tested to 4.5 V at 55 • C. Capacity fade during cycling, charge-transfer resistance (R ct) before and after cycling as well as gas evolution during formation and also during cycling were examined and compared head-to-head. Of all the electrolytes tested, triallyl phosphate containing electrolytes including 2% vinylene carbonate + 2% triallyl phosphate in 1 M LiPF 6 sulfolane:ethyl methyl carbonate and 2% prop-1-ene sultone + 2% triallyl phosphate in 1M LiPF 6 ethylene carbonate:ethyl methyl carbonate electrolytes showed the best capacity retention, the least impedance growth and manageable amounts of gas evolution during long-term cycling. Pyridine boron trifluoride-based additives also showed excellent cycling performance but cells with those additives had higher gas evolution during cycling. Cells containing fluorinated electrolytes had similar cycling performance to cells containing 2% prop-1-ene sultone in ethylene carbonate:ethyl methyl carbonate electrolyte but with much more gas evolution and higher impedance after long-term cycling. The results suggest that electrolytes containing the additives triallyl phosphate or pyridine boron trifluoride may be promising for high voltage Li-ion cells at elevated temperature.
Journal of Power Sources Volume 328, 1 October 2016, Pages 433–442, Aug 17, 2016
Describes several Lewis acid-base adducts as electrolyte additives. Pyridine boron trifluoride is... more Describes several Lewis acid-base adducts as electrolyte additives. Pyridine boron trifluoride is the best adduct studied here. Pyridine boron trifluoride improved storage, impedance and long-term cycling. a b s t r a c t Three complexes with boron trifluoride (BF 3) as the Lewis acid and different Lewis bases were synthesized and used as electrolyte additives in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite and Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 / graphite pouch cells. Lewis acid-base adducts with a boron-oxygen (BeO) bond were trimethyl phosphate boron trifluoride (TMP-BF) and triphenyl phosphine oxide boron trifluoride (TPPO-BF). These were compared to pyridine boron trifluoride (PBF) which has a boron-nitrogen (BeN) bond. The experimental results showed that cells with PBF had the least voltage drop during storage at 4.2 V, 4.4 V and 4.7 V at 40 C and the best capacity retention during long-term cycling at 55 C compared to cells with the other additives. Charge-hold-discharge cycling combined with simultaneous electrochemical impedance spectroscopy measurements showed that impedance growth in TMP-BF and TPPO-BF containing cells was faster than cells containing 2%PBF, suggesting that PBF is useful for impedance control at high voltages (>4.4 V). XPS analysis of the SEI films highlighted a specific reactivity of the PBF-derived SEI species that apparently hinders the degradation of both LiPF 6 and solvent during formation and charge-hold-discharge cycling. The modified SEI films may explain the improved impedance, the smaller voltage drop during storage and the improved capacity retention during cycling of cells containing the PBF additive.
Three Lewis acid-base adducts including pyridine boron trifluoride (PBF), pyridine phosphorus pen... more Three Lewis acid-base adducts including pyridine boron trifluoride (PBF), pyridine phosphorus pentafluoride (PPF) and pyridine sulfur trioxide (PSO) were used as electrolyte additives in Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 /graphite pouch cells (NMC532/graphite). PBF, PPF and PSO have a common Lewis base (pyridine) but have different Lewis acids. Experiments included ultra-high precision coulometry (UHPC), electrochemical impedance spectroscopy (EIS), gas evolution, long-term continuous charge-discharge cycling and charge-hold-discharge cycling. The results showed that cells containing PBF and PPF had the smallest voltage drop during storage at 4.5 V and at 60 • C compared to cells with PSO and other additives such as triallyl phosphate (TAP) and vinylene carbonate (VC). UHPC results showed that the coulombic efficiency (CE) of the cells could be improved by using PBF or PPF either singly or in combination with other additives. The charge-hold-discharge cycling protocol can be used to distinguish the difference between additives relatively quickly compared to continuous cycling. Cells with 2% PBF had the best capacity retention compared to cells with the other additives at 40 • C.
Journal of Power Sources Volume 327, 30 September 2016, Pages 145–150, Jul 22, 2016
Li[Ni 0.8 Mn 0.1 Co 0.1 ]O 2 shows serious reactivity with electrolyte above 110 C. Li[Ni 0.4 Mn ... more Li[Ni 0.8 Mn 0.1 Co 0.1 ]O 2 shows serious reactivity with electrolyte above 110 C. Li[Ni 0.4 Mn 0.4 Co 0.2 ]O 2 shows the least reactivity with electrolyte at high temperature. Li[Ni 0.6 Mn 0.2 Co 0.2 ]O 2 and Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 show intermediate reactivity. This work identifies important trade-offs between Li[Ni x Mn y Co z ]O 2 grades. a b s t r a c t The reactivity between charged Li[Ni x Mn y Co z ]O 2 (NMC, with x þ y þ z ¼ 1, x:y:z ¼ 1:1:1 (NMC111), 4:4:2 (NMC442), 5:3:2 (NMC532), 6:2:2 (NMC622) and 8:1:1 (NMC811)) and traditional carbonate-based electrolytes at elevated temperatures was systematically studied using accelerating rate calorimetry (ARC). The ARC results showed that the upper cutoff potential and NMC composition strongly affect the thermal stability of the various NMC grades when traditional carbonate-based electrolyte was used. Although higher cutoff potential and higher Ni content can help increase the energy density of lithium ion cells, these factors generally increase the reactivity between charged NMC and electrolyte at elevated temperatures. It is hoped that this report can be used to help guide the wise selection of NMC grade and upper cutoff potential to achieve high energy density Li-ion cells without seriously compromising cell safety.
J. Electrochem. Soc. 2016 volume 163, issue 5, A773-A780
The use of electrolyte additives to form a passive solid-electrolyte interphase (SEI) at one or b... more The use of electrolyte additives to form a passive solid-electrolyte interphase (SEI) at one or both electrodes is a common method for improving lithium-ion cell lifetime and performance. This work follows the chemical and electrochemical processes involved in SEI formation on graphite electrodes for two Lewis acid-base adducts, pyridine boron trifluoride (PBF) and pyridine phosphorus pentafluoride (PPF). The combination of experimental methods (electrochemistry, in situ volumetric measurements, gas chromatography , isothermal microcalorimetry, and X-ray photoelectron spectroscopy) with quantum chemistry models (density functional theory) provides new insight into the interfacial chemistry. PBF and PPF are reduced at ∼1.3 V vs. Li/Li + and ∼1.4 V, respectively. This is followed by radical coupling to form 4,4-bipyridine adducts, hydrogen transfer to ethylene carbonate solvent molecules, and reduction of the solvent to produce lithium ethyl carbonate. The reduced bipyridine adducts, Li 2 (PBF) 2 and Li 2 (PPF) 2 , are shown to compose part of the SEI at the negative electrode surface.
Journal of Power Sources Volume 307, 1 March 2016, Pages 340–350
A fluorinated electrolyte mixture, containing 1 M LiPF6/fluoroethylene carbonate:bis (2,2,2-trifl... more A fluorinated electrolyte mixture, containing 1 M LiPF6/fluoroethylene carbonate:bis (2,2,2-trifluoroethyl) carbonate (1:1 w:w) with prop-1-ene-1,3-sultone as an electrolyte additive exhibited promising cycling and storage performance in Li(Ni0.4Mn0.4Co0.2)O2/graphite pouch type Li-ion cells tested to 4.5 V. The prop-1-ene-1,3-sultone additive was added to help control gas evolution in the fluorinated electrolyte cells, which was improved but still problematic even with the additive. Cells with the fluorinated electrolyte demonstrated higher impedance in early cycles compared to cells with carbonate solvents and state of the art additives. Symmetric cells were used to show this high impedance originated at the negative electrode/electrolyte interface. Nevertheless, in charge–discharge cycling tests to 4.5 V, cells with the fluorinated electrolyte and 1, 2 or 3% prop-1-ene-1,3-sultone additive, outperformed all non-fluorinated electrolytes with all additives tested. With further work, these, or other fluorinated carbonates, coupled with appropriate additives, may represent a viable path to NMC/graphite cells that can operate to 4.5 V and above.
J. Electrochem. Soc. 2016 volume 163, issue 3, A546-A551
When NMC/graphite Li-ion cells are operated at elevated temperature or at a cutoff potential abov... more When NMC/graphite Li-ion cells are operated at elevated temperature or at a cutoff potential above 4.2 V, electrolyte oxidation becomes increasingly severe leading to gaseous products and other oxidized species. These generated gas products and oxidized species can migrate to, and then interact with, the negative electrode. A variety of cell formats (pouch cells, symmetric cells and coin cells) as well as pouch bags, containing only a delithiated positive electrode or a lithiated negative electrode, were used to investigate electrode/electrode interactions. Open circuit potential measurements during high temperature storage, ex-situ measurements of gas volume produced versus time, gas chromatography-mass spectrometry (GC-MS) of the gases produced and electrochemical impedance spectroscopy (EIS) of the electrodes versus time were performed. During storage at 60°C, pouch bags containing only a lithiated negative electrode and electrolyte produced no gas while charged full pouch cells produced some gas and pouch bags containing only a delithiated positive electrode and electrolyte produced a significant amount of gas. The predominant gas produced in the positive electrode pouch bags was CO2 while virtually no CO2 was detected in the gases evolved in the charged full cell, suggesting that the negative electrode in the full cell consumes CO2 generated at the charged positive electrode. In addition, the impedance of the surface film on the charged positive electrodes in the pouch bags increased at least three times more than the positive electrodes in the charged pouch cells, even though they were both in contact with electrolyte for the same period of time. These impedance results suggest that oxidized species created at the positive electrode in the pouch bag remain in the vicinity of the positive electrode and create a high impedance film possibly a rock salt surface layer, while the same species migrate to the negative and are “consumed” in the pouch cell where the impedance of the positive electrode remains small. These interactions are apparently essential for the health of a NMC/graphite Li-ion pouch cell when operated at an elevated temperature or at a cutoff voltage above 4.2 V.
Journal of Power Sources 2016, Volume 306, Pages 233–240, Dec 17, 2015
Three different cycling protocols including “continuous-cycling”, “barn-charge” and “cycle-store”... more Three different cycling protocols including “continuous-cycling”, “barn-charge” and “cycle-store” were applied with an ultra high precision charger to Li[Ni0.42Mn0.42Co0.16]O2/graphite and/or Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells tested using different upper cutoff potentials. The barn-charge and cycle-store protocols were designed so that cells stay at high potential for a larger fraction of their testing time compared to continuous cycling. For cells tested to 4.2, 4.4 or 4.5 V, the greater the fraction of testing time spent at high potential, the lower the coulombic efficiency and the greater the charge endpoint capacity slippage rate, with the effects being more severe at higher potential. These results confirm that Li[Ni0.42Mn0.42Co0.16]O2/graphite and Li[Ni1/3Mn1/3Co1/3]O2/graphite Li-ion cells which are charged and then left at high potential (>4.4 V) for extended periods of time will have much shorter calendar and cycle life compared to those that are continuously cycled as has been recently reported in long-term test results.
Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonat... more Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonate (EC), fluoroethylene carbonate (FEC), or EC with 15% FEC (EC:FEC), extracted from cells and analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES). All of the electrolytes generate an SEI which is integrated with Si containing species. The EC and EC:FEC electrolytes result in the generation of LixSiOy after the first cycle while LixSiOy is only observed after five cycles for the FEC electrolyte. The SEI initially generated from the EC electrolyte is primarily composed of lithium ethylene dicarbonate (LEDC) and LiF. However, after five cycles, the composition changes, especially near the surface of silicon because of decomposition of the LEDC. The SEI generated from the EC:FEC electrolytes contains LEDC, LiF, and poly(FEC) and small changes are observed upon additional cycling. The SEI generated with the FEC electrolyte contains LiF and poly(FEC) and small changes are observed upon additional cycling. The stability of the SEI correlates with the observed capacity retention of the cells.
Journal of Power Sources Volume 299, 20 December 2015, Pages 130–138, Sep 5, 2015
Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite... more Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite pouch cells were examined with and without electrolyte additives using the ultra high precision charger at Dalhousie University, electrochemical impedance spectroscopy, gas evolution measurements and “cycle-store” tests. The electrolyte additives tested were vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), pyridine-boron trifluoride (PBF), 2% PES + 1% methylene methanedisulfonate (MMDS) + 1% tris(trimethylsilyl) phosphite (TTSPi) and 0.5% pyrazine di-boron trifluoride (PRZ) + 1% MMDS. The charge end-point capacity slippage, capacity fade, coulombic efficiency, impedance change during cycling, gas evolution and voltage drop during “cycle-store” testing were compared to gain an understanding of the effects of these promising electrolyte additives or additive combinations on the different types of pouch cells. It is hoped that this report can be used as a guide or reference for the wise choice of electrolyte additives in Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite pouch cells and also to show the shortcomings of particular positive electrode compositions.
The novel electrolyte additives pyridine boron trifluoride, pyrazine di-boron trifluoride and tri... more The novel electrolyte additives pyridine boron trifluoride, pyrazine di-boron trifluoride and triazine tri-boron trifluoride were compared in Li[Ni1/3Mn1/3Co1/3]O2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells. This series of additives allowed the Lewis base:BF3 ratio to be systematically varied. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC) or prop-1-ene-1,3-sultone (PES). Increasing the BF3 content on the additive can reduce the impedance of cells during charge-discharge cycling. However the coulombic efficiency, charge endpoint capacity slippage and long-term cycling behavior of these pouch cells was not improved by increasing the content of BF3 in the additives.
Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte addit... more Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the structure of the solid electrolyte interface (SEI). The structure of the SEI has been investigated via ex-situ surface analysis including X-ray Photoelectron spectroscopy (XPS), Hard XPS (HAXPES), Infrared spectroscopy (IR) and transmission electron microscopy (TEM). The components of the SEI have been further investigated via nuclear magnetic resonance (NMR) spectroscopy of D2O extractions. The SEI generated on the BF-graphite anode with a standard electrolyte (1.2 M LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 3/7 (v/v)) is composed primarily of lithium alkyl carbonates (LAC) and LiF. Incorporation of VC (3% wt) results in the generation of a thinner SEI composed of Li2CO3, poly(VC), LAC, and LiF. Incorporation of VC inhibits the generation of LAC and LiF. Incorporation of FEC (3% wt) also results in the generation of a thinner SEI composed of Li2CO3, poly(FEC), LAC, and LiF. The concentration of poly(FEC) is lower than the concentration of poly(VC) and the generation of LAC is inhibited in the presence of FEC. The SEI appears to be a homogeneous film for all electrolytes investigated.
J. Electrochem. Soc. 2015 volume 162, issue 9, A1693-A1701 , Jun 11, 2015
Pyridine Boron Trifluoride (PBF)-based electrolyte additive blends for Li[Ni0.42Mn0.42Co0.16]O2 (... more Pyridine Boron Trifluoride (PBF)-based electrolyte additive blends for Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells have been systematically investigated. Other additives investigated in the blends included methylene methane disulfonate (MMDS) and ethylene sulfate (DTD). Ultra-high precision coulometry (UHPC), electrochemical impedance spectroscopy (EIS), gas evolution and long-term cycling experiments were made. The results obtained in this work indicate that MMDS and DTD have good compatibility with PBF-type additives and some of their combinations showed significant improvements in coulombic efficiency, reducing impedance and maintaining good capacity retention during high-voltage cycling compared to PBF-type additives alone.
J. Electrochem. Soc. 2015 volume 162, issue 7, A1186-A1195 , Mar 31, 2015
A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and su... more A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and successfully synthesized. The synthesis is very simple: a pyridine derivative is mixed with boron trifluoride dissolved in diethyl ether to yield a solid crystalline product. The effect of Pyridine-Boron Trifluoride (PBF) and its derivatives have been thoroughly evaluated in Li[Ni1/3Mn1/3Co1/3]O2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells. Evaluation experiments, including high voltage storage, gas production, electrochemical impedance spectroscopy measurements, ultra high precision cycling and long-term cycling were carried out on cells containing the novel additives. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC), prop-1-ene sultone (PES), methylene methane disulfonate (MMDS), tris(-trimethyl-silyl)-phosphite (TTSPi) and triallyl phosphate (TAP). The PBF additives are competitive with all known additives in NMC/graphite cells, which, combined with their low cost and facile synthesis, suggest this series of novel additives will be useful in high-voltage/high-temperature lithium ion battery applications. The PBF additives yield cells which show excellent capacity retention and maintain low impedance during high voltage cycling, in contrast to cells containing VC.
J. Electrochem. Soc. 2015 volume 162, issue 7, A1186-A1195 , Mar 31, 2015
A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and su... more A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and successfully synthesized. The synthesis is very simple: a pyridine derivative is mixed with boron trifluoride dissolved in diethyl ether to yield a solid crystalline product. The effect of Pyridine-Boron Trifluoride (PBF) and its derivatives have been thoroughly evaluated in Li[Ni1/3Mn1/3Co1/3]O2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells. Evaluation experiments, including high voltage storage, gas production, electrochemical impedance spectroscopy measurements, ultra high precision cycling and long-term cycling were carried out on cells containing the novel additives. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC), prop-1-ene sultone (PES), methylene methane disulfonate (MMDS), tris(-trimethyl-silyl)-phosphite (TTSPi) and triallyl phosphate (TAP). The PBF additives are competitive with all known additives in NMC/graphite cells, which, combined with their low cost and facile synthesis, suggest this series of novel additives will be useful in high-voltage/high-temperature lithium ion battery applications. The PBF additives yield cells which show excellent capacity retention and maintain low impedance during high voltage cycling, in contrast to cells containing VC.
Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonat... more Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonates solvents used in lithium ion battery electrolytes. The reaction precipitates have been analyzed by IR-ATR and solution NMR spectroscopy and the evolved gases have been analyzed by GC-MS. The reduction products of ethylene carbonate and propylene carbonate are lithium ethylene dicarbonate and ethylene and lithium propylene dicarbonate and propylene, respectively. The reduction products of diethyl and dimethyl carbonate are lithium ethyl carbonate and ethane and lithium methyl carbonate and methane, respectively. Lithium carbonate is not observed as a reduction product.
Silicon (Si) is a promising candidate for lithium ion battery anodes because of its high theoreti... more Silicon (Si) is a promising candidate for lithium ion battery anodes because of its high theoretical capacity. However, the large volume changes during lithiation/delithiation cycles result in pulverization of Si, leading to rapid fading of capacity. Here, we report a simple fabrication technique that is designed to overcome many of the limitations that deter more widespread adoption of Si based anodes. We confine Si nanoparticles in the oil phase of an oil-in-water emulsion stabilized by carbon black (CB). These CB nanoparticles are both oil- and water-wettable. The hydrophilic/hydrophobic balance for the CB nanoparticles also causes them to form a network in the continuous aqueous phase. Upon drying this emulsion on a current collector, the CB particles located at the surfaces of the emulsion droplets form mesoporous cages that loosely encapsulate the Si particles that were in the oil. The CB particles that were in the aqueous phase form a conducting network connected to the CB cages. The space within the cages allows for Si particle expansion without transmitting stresses to the surrounding carbon network. Half-cell experiments using this Si/CB anode architecture show a specific capacity of ∼1300 mAh/g Si + C and a Coulombic efficiency of 97.4% after 50 cycles. Emulsion-templating is a simple, inexpensive processing strategy that directs Si and conducts CB particles to desired spatial locations for superior performance of anodes in lithium ion batteries.
J. Electrochem. Soc. 2014 volume 161, issue 6, A1001-A1006, May 1, 2014
A comparative investigation of the different lithium salts on formation of the solid electrolyte ... more A comparative investigation of the different lithium salts on formation of the solid electrolyte interface (SEI) on binder free graphite anodes for lithium ion batteries has been conducted. The electrolytes investigated include 1 M LiPF6, LiBF4, LiTFSI, LiFSI, LiDFOB or LiBOB dissolved in ethylene carbonate (EC). The SEI has been investigated via a combination of spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) allows direct observation of the SEI formed from the different electrolytes. Nuclear magnetic resonance (NMR) spectroscopy of D2O extracts are utilized to characterize the soluble species of SEI. XPS and FTIR provide additional elemental and functional group information for the SEI components. The SEI for all electrolytes contains lithium ethylene dicarbonate (LEDC), the primary reduction product of EC. In addition, the SEI for all electrolytes contain LiF except for the SEI generated from the LiBOB electrolyte. The SEI generated in the presence of LiBOB or LiDFOB electrolytes contain multiple oxalate containing species, including lithium oxalate (Li2C2O4), and borates.
J. Phys. Chem. C, 2013, 117 (48), pp 25381–25389, Nov 13, 2013
An investigation of the interrelationship of cycling performance, solution structure, and electro... more An investigation of the interrelationship of cycling performance, solution structure, and electrode surface film structure has been conducted for electrolytes composed of different concentrations of LiPF6 in propylene carbonate (PC) with a binder-free (BF) graphite electrode. Varying the concentration of LiPF6 changes the solution structure, altering the predominant mechanism of electrolyte reduction at the electrode interface. The change in mechanism results in a change in the structure of the solid electrolyte interface (SEI) and the reversible cycling of the cell. At low concentrations of LiPF6 in PC (1.2 M), electrochemical cycling and cyclic voltammetry (CV) of BF graphite electrodes reveal continuous electrolyte reduction and no lithiation/delithiation of the graphite. The solution structure is dominated by solvent-separated ion pairs (Li+(PC)4//PF6–), and the primary reduction product of the electrolyte is lithium propylene dicarbonate (LPDC). At high concentrations of LiPF6 in PC (3.0–3.5 M), electrochemical cycling and CV reveal reversible lithiation/delithiation of the graphite electrode. The solution structure is dominated by contact ion pairs (Li+(PC)3PF6–), and the primary reduction product of the electrolyte is LiF.
J. Electrochem. Soc. 2017 volume 164, issue 4, A655-A665, Feb 2, 2017
Many literature reports show that layered Li-Ni-Mn-Co oxides (NMC) have a surface reconstruction ... more Many literature reports show that layered Li-Ni-Mn-Co oxides (NMC) have a surface reconstruction to a rocksalt (Fm3m) structure which is claimed to be responsible for the increase in cell impedance during high voltage cycling. It is important to determine if appropriate electrolyte additives can suppress the surface reconstructions of NMC materials. LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811)/Graphite pouch cells with different electrolyte additives and different upper cutoff potentials were charge-discharge cycled and the electrodes were recovered for z-contrast scanning transmission electron microscope (STEM) studies. It was found that there was no significant surface layer growth for cells cycled between 2.8 and 4.1 V. For cells with an upper cutoff voltage of 4.3 V, the electrodes from cells with control electrolyte (no additives) showed the thickest surface layer. The electrolyte additives vinylene carbonate (VC) and prop-1-ene-1,3-sultone (PES) were found to suppress the growth of the surface layer. However, cells with PES showed a more rapid capacity fade than control cells or cells with 2% VC showing that, at least for NMC811/graphite cells with PES or VC additives, failure cannot only be solely ascribed to a growing rocksalt surface layer. Other processes, for example associated with electrolyte oxidation, are believed to be responsible for failure. High energy density lithium-ion batteries that are cheaper, safer and with longer lifetimes need to be developed in order to meet the increasing demand for applications such as electric vehicles. LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) can deliver a high capacity of ∼200 mAh/g with an average discharge voltage of ∼3.8 V (vs. Li+/Li), making it a promising positive electrode material for high energy density lithium-ion batteries. 1 However, electrochemical tests of NMC811 from half cells and full cells show poor cycling performance when charged to voltages above 4.2 V. 2 In-situ and ex-situ X-ray diffraction showed that there are no significant irreversible structural changes in the bulk of the material during charge-discharge cycling. Instead, the parasitic reactions between the electrolyte and the surface of the positive electrode particles at high voltages were suggested to be the cause of the failure of cells cycled above 4.2 V. 2 Layered NMC materials have a hexagonal layered structure (α-NaFeO 2-type structure described in the R ¯ m space group), where Li and transition metal atoms form alternating layers between oxygen layers and Li atoms have a 2-D diffusion path. 3–5 Recently, Lin et al. 6 showed that the surface of LiNi 0.42 Mn 0.42 Co 0.16 O 2 (NMC442) went through a structural reconstruction from layered (R ¯ m) to rocksalt (Fm3m). In that transition, transition metal ions migrated to the lithium layers with a possible loss of Li and O from the surface of the structure. This was cited as one of the causes of a significant increase in cell impedance under high voltage cycling conditions. This surface reconstruction phenomenon was also observed in many other reports about NMC and Li[Ni 0.80 Co 0.15 Al 0.05 ]O 2 positive electrode materials where the surface reconstruction was ascribed to be a result of interactions between the positive electrode surface and the electrolyte. 6–22
The long-term cycling behavior of 24 promising electrolyte blends were systematically studied in ... more The long-term cycling behavior of 24 promising electrolyte blends were systematically studied in LaPO 4-coated Li(Ni 0.4 Mn 0.4 Co 0.2)O 2 /graphite pouch type Li-ion cells tested to 4.5 V at 55 • C. Capacity fade during cycling, charge-transfer resistance (R ct) before and after cycling as well as gas evolution during formation and also during cycling were examined and compared head-to-head. Of all the electrolytes tested, triallyl phosphate containing electrolytes including 2% vinylene carbonate + 2% triallyl phosphate in 1 M LiPF 6 sulfolane:ethyl methyl carbonate and 2% prop-1-ene sultone + 2% triallyl phosphate in 1M LiPF 6 ethylene carbonate:ethyl methyl carbonate electrolytes showed the best capacity retention, the least impedance growth and manageable amounts of gas evolution during long-term cycling. Pyridine boron trifluoride-based additives also showed excellent cycling performance but cells with those additives had higher gas evolution during cycling. Cells containing fluorinated electrolytes had similar cycling performance to cells containing 2% prop-1-ene sultone in ethylene carbonate:ethyl methyl carbonate electrolyte but with much more gas evolution and higher impedance after long-term cycling. The results suggest that electrolytes containing the additives triallyl phosphate or pyridine boron trifluoride may be promising for high voltage Li-ion cells at elevated temperature.
Journal of Power Sources Volume 328, 1 October 2016, Pages 433–442, Aug 17, 2016
Describes several Lewis acid-base adducts as electrolyte additives. Pyridine boron trifluoride is... more Describes several Lewis acid-base adducts as electrolyte additives. Pyridine boron trifluoride is the best adduct studied here. Pyridine boron trifluoride improved storage, impedance and long-term cycling. a b s t r a c t Three complexes with boron trifluoride (BF 3) as the Lewis acid and different Lewis bases were synthesized and used as electrolyte additives in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite and Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 / graphite pouch cells. Lewis acid-base adducts with a boron-oxygen (BeO) bond were trimethyl phosphate boron trifluoride (TMP-BF) and triphenyl phosphine oxide boron trifluoride (TPPO-BF). These were compared to pyridine boron trifluoride (PBF) which has a boron-nitrogen (BeN) bond. The experimental results showed that cells with PBF had the least voltage drop during storage at 4.2 V, 4.4 V and 4.7 V at 40 C and the best capacity retention during long-term cycling at 55 C compared to cells with the other additives. Charge-hold-discharge cycling combined with simultaneous electrochemical impedance spectroscopy measurements showed that impedance growth in TMP-BF and TPPO-BF containing cells was faster than cells containing 2%PBF, suggesting that PBF is useful for impedance control at high voltages (>4.4 V). XPS analysis of the SEI films highlighted a specific reactivity of the PBF-derived SEI species that apparently hinders the degradation of both LiPF 6 and solvent during formation and charge-hold-discharge cycling. The modified SEI films may explain the improved impedance, the smaller voltage drop during storage and the improved capacity retention during cycling of cells containing the PBF additive.
Three Lewis acid-base adducts including pyridine boron trifluoride (PBF), pyridine phosphorus pen... more Three Lewis acid-base adducts including pyridine boron trifluoride (PBF), pyridine phosphorus pentafluoride (PPF) and pyridine sulfur trioxide (PSO) were used as electrolyte additives in Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 /graphite pouch cells (NMC532/graphite). PBF, PPF and PSO have a common Lewis base (pyridine) but have different Lewis acids. Experiments included ultra-high precision coulometry (UHPC), electrochemical impedance spectroscopy (EIS), gas evolution, long-term continuous charge-discharge cycling and charge-hold-discharge cycling. The results showed that cells containing PBF and PPF had the smallest voltage drop during storage at 4.5 V and at 60 • C compared to cells with PSO and other additives such as triallyl phosphate (TAP) and vinylene carbonate (VC). UHPC results showed that the coulombic efficiency (CE) of the cells could be improved by using PBF or PPF either singly or in combination with other additives. The charge-hold-discharge cycling protocol can be used to distinguish the difference between additives relatively quickly compared to continuous cycling. Cells with 2% PBF had the best capacity retention compared to cells with the other additives at 40 • C.
Journal of Power Sources Volume 327, 30 September 2016, Pages 145–150, Jul 22, 2016
Li[Ni 0.8 Mn 0.1 Co 0.1 ]O 2 shows serious reactivity with electrolyte above 110 C. Li[Ni 0.4 Mn ... more Li[Ni 0.8 Mn 0.1 Co 0.1 ]O 2 shows serious reactivity with electrolyte above 110 C. Li[Ni 0.4 Mn 0.4 Co 0.2 ]O 2 shows the least reactivity with electrolyte at high temperature. Li[Ni 0.6 Mn 0.2 Co 0.2 ]O 2 and Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 show intermediate reactivity. This work identifies important trade-offs between Li[Ni x Mn y Co z ]O 2 grades. a b s t r a c t The reactivity between charged Li[Ni x Mn y Co z ]O 2 (NMC, with x þ y þ z ¼ 1, x:y:z ¼ 1:1:1 (NMC111), 4:4:2 (NMC442), 5:3:2 (NMC532), 6:2:2 (NMC622) and 8:1:1 (NMC811)) and traditional carbonate-based electrolytes at elevated temperatures was systematically studied using accelerating rate calorimetry (ARC). The ARC results showed that the upper cutoff potential and NMC composition strongly affect the thermal stability of the various NMC grades when traditional carbonate-based electrolyte was used. Although higher cutoff potential and higher Ni content can help increase the energy density of lithium ion cells, these factors generally increase the reactivity between charged NMC and electrolyte at elevated temperatures. It is hoped that this report can be used to help guide the wise selection of NMC grade and upper cutoff potential to achieve high energy density Li-ion cells without seriously compromising cell safety.
J. Electrochem. Soc. 2016 volume 163, issue 5, A773-A780
The use of electrolyte additives to form a passive solid-electrolyte interphase (SEI) at one or b... more The use of electrolyte additives to form a passive solid-electrolyte interphase (SEI) at one or both electrodes is a common method for improving lithium-ion cell lifetime and performance. This work follows the chemical and electrochemical processes involved in SEI formation on graphite electrodes for two Lewis acid-base adducts, pyridine boron trifluoride (PBF) and pyridine phosphorus pentafluoride (PPF). The combination of experimental methods (electrochemistry, in situ volumetric measurements, gas chromatography , isothermal microcalorimetry, and X-ray photoelectron spectroscopy) with quantum chemistry models (density functional theory) provides new insight into the interfacial chemistry. PBF and PPF are reduced at ∼1.3 V vs. Li/Li + and ∼1.4 V, respectively. This is followed by radical coupling to form 4,4-bipyridine adducts, hydrogen transfer to ethylene carbonate solvent molecules, and reduction of the solvent to produce lithium ethyl carbonate. The reduced bipyridine adducts, Li 2 (PBF) 2 and Li 2 (PPF) 2 , are shown to compose part of the SEI at the negative electrode surface.
Journal of Power Sources Volume 307, 1 March 2016, Pages 340–350
A fluorinated electrolyte mixture, containing 1 M LiPF6/fluoroethylene carbonate:bis (2,2,2-trifl... more A fluorinated electrolyte mixture, containing 1 M LiPF6/fluoroethylene carbonate:bis (2,2,2-trifluoroethyl) carbonate (1:1 w:w) with prop-1-ene-1,3-sultone as an electrolyte additive exhibited promising cycling and storage performance in Li(Ni0.4Mn0.4Co0.2)O2/graphite pouch type Li-ion cells tested to 4.5 V. The prop-1-ene-1,3-sultone additive was added to help control gas evolution in the fluorinated electrolyte cells, which was improved but still problematic even with the additive. Cells with the fluorinated electrolyte demonstrated higher impedance in early cycles compared to cells with carbonate solvents and state of the art additives. Symmetric cells were used to show this high impedance originated at the negative electrode/electrolyte interface. Nevertheless, in charge–discharge cycling tests to 4.5 V, cells with the fluorinated electrolyte and 1, 2 or 3% prop-1-ene-1,3-sultone additive, outperformed all non-fluorinated electrolytes with all additives tested. With further work, these, or other fluorinated carbonates, coupled with appropriate additives, may represent a viable path to NMC/graphite cells that can operate to 4.5 V and above.
J. Electrochem. Soc. 2016 volume 163, issue 3, A546-A551
When NMC/graphite Li-ion cells are operated at elevated temperature or at a cutoff potential abov... more When NMC/graphite Li-ion cells are operated at elevated temperature or at a cutoff potential above 4.2 V, electrolyte oxidation becomes increasingly severe leading to gaseous products and other oxidized species. These generated gas products and oxidized species can migrate to, and then interact with, the negative electrode. A variety of cell formats (pouch cells, symmetric cells and coin cells) as well as pouch bags, containing only a delithiated positive electrode or a lithiated negative electrode, were used to investigate electrode/electrode interactions. Open circuit potential measurements during high temperature storage, ex-situ measurements of gas volume produced versus time, gas chromatography-mass spectrometry (GC-MS) of the gases produced and electrochemical impedance spectroscopy (EIS) of the electrodes versus time were performed. During storage at 60°C, pouch bags containing only a lithiated negative electrode and electrolyte produced no gas while charged full pouch cells produced some gas and pouch bags containing only a delithiated positive electrode and electrolyte produced a significant amount of gas. The predominant gas produced in the positive electrode pouch bags was CO2 while virtually no CO2 was detected in the gases evolved in the charged full cell, suggesting that the negative electrode in the full cell consumes CO2 generated at the charged positive electrode. In addition, the impedance of the surface film on the charged positive electrodes in the pouch bags increased at least three times more than the positive electrodes in the charged pouch cells, even though they were both in contact with electrolyte for the same period of time. These impedance results suggest that oxidized species created at the positive electrode in the pouch bag remain in the vicinity of the positive electrode and create a high impedance film possibly a rock salt surface layer, while the same species migrate to the negative and are “consumed” in the pouch cell where the impedance of the positive electrode remains small. These interactions are apparently essential for the health of a NMC/graphite Li-ion pouch cell when operated at an elevated temperature or at a cutoff voltage above 4.2 V.
Journal of Power Sources 2016, Volume 306, Pages 233–240, Dec 17, 2015
Three different cycling protocols including “continuous-cycling”, “barn-charge” and “cycle-store”... more Three different cycling protocols including “continuous-cycling”, “barn-charge” and “cycle-store” were applied with an ultra high precision charger to Li[Ni0.42Mn0.42Co0.16]O2/graphite and/or Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells tested using different upper cutoff potentials. The barn-charge and cycle-store protocols were designed so that cells stay at high potential for a larger fraction of their testing time compared to continuous cycling. For cells tested to 4.2, 4.4 or 4.5 V, the greater the fraction of testing time spent at high potential, the lower the coulombic efficiency and the greater the charge endpoint capacity slippage rate, with the effects being more severe at higher potential. These results confirm that Li[Ni0.42Mn0.42Co0.16]O2/graphite and Li[Ni1/3Mn1/3Co1/3]O2/graphite Li-ion cells which are charged and then left at high potential (>4.4 V) for extended periods of time will have much shorter calendar and cycle life compared to those that are continuously cycled as has been recently reported in long-term test results.
Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonat... more Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonate (EC), fluoroethylene carbonate (FEC), or EC with 15% FEC (EC:FEC), extracted from cells and analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES). All of the electrolytes generate an SEI which is integrated with Si containing species. The EC and EC:FEC electrolytes result in the generation of LixSiOy after the first cycle while LixSiOy is only observed after five cycles for the FEC electrolyte. The SEI initially generated from the EC electrolyte is primarily composed of lithium ethylene dicarbonate (LEDC) and LiF. However, after five cycles, the composition changes, especially near the surface of silicon because of decomposition of the LEDC. The SEI generated from the EC:FEC electrolytes contains LEDC, LiF, and poly(FEC) and small changes are observed upon additional cycling. The SEI generated with the FEC electrolyte contains LiF and poly(FEC) and small changes are observed upon additional cycling. The stability of the SEI correlates with the observed capacity retention of the cells.
Journal of Power Sources Volume 299, 20 December 2015, Pages 130–138, Sep 5, 2015
Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite... more Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite pouch cells were examined with and without electrolyte additives using the ultra high precision charger at Dalhousie University, electrochemical impedance spectroscopy, gas evolution measurements and “cycle-store” tests. The electrolyte additives tested were vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), pyridine-boron trifluoride (PBF), 2% PES + 1% methylene methanedisulfonate (MMDS) + 1% tris(trimethylsilyl) phosphite (TTSPi) and 0.5% pyrazine di-boron trifluoride (PRZ) + 1% MMDS. The charge end-point capacity slippage, capacity fade, coulombic efficiency, impedance change during cycling, gas evolution and voltage drop during “cycle-store” testing were compared to gain an understanding of the effects of these promising electrolyte additives or additive combinations on the different types of pouch cells. It is hoped that this report can be used as a guide or reference for the wise choice of electrolyte additives in Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite pouch cells and also to show the shortcomings of particular positive electrode compositions.
The novel electrolyte additives pyridine boron trifluoride, pyrazine di-boron trifluoride and tri... more The novel electrolyte additives pyridine boron trifluoride, pyrazine di-boron trifluoride and triazine tri-boron trifluoride were compared in Li[Ni1/3Mn1/3Co1/3]O2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells. This series of additives allowed the Lewis base:BF3 ratio to be systematically varied. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC) or prop-1-ene-1,3-sultone (PES). Increasing the BF3 content on the additive can reduce the impedance of cells during charge-discharge cycling. However the coulombic efficiency, charge endpoint capacity slippage and long-term cycling behavior of these pouch cells was not improved by increasing the content of BF3 in the additives.
Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte addit... more Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the structure of the solid electrolyte interface (SEI). The structure of the SEI has been investigated via ex-situ surface analysis including X-ray Photoelectron spectroscopy (XPS), Hard XPS (HAXPES), Infrared spectroscopy (IR) and transmission electron microscopy (TEM). The components of the SEI have been further investigated via nuclear magnetic resonance (NMR) spectroscopy of D2O extractions. The SEI generated on the BF-graphite anode with a standard electrolyte (1.2 M LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 3/7 (v/v)) is composed primarily of lithium alkyl carbonates (LAC) and LiF. Incorporation of VC (3% wt) results in the generation of a thinner SEI composed of Li2CO3, poly(VC), LAC, and LiF. Incorporation of VC inhibits the generation of LAC and LiF. Incorporation of FEC (3% wt) also results in the generation of a thinner SEI composed of Li2CO3, poly(FEC), LAC, and LiF. The concentration of poly(FEC) is lower than the concentration of poly(VC) and the generation of LAC is inhibited in the presence of FEC. The SEI appears to be a homogeneous film for all electrolytes investigated.
J. Electrochem. Soc. 2015 volume 162, issue 9, A1693-A1701 , Jun 11, 2015
Pyridine Boron Trifluoride (PBF)-based electrolyte additive blends for Li[Ni0.42Mn0.42Co0.16]O2 (... more Pyridine Boron Trifluoride (PBF)-based electrolyte additive blends for Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells have been systematically investigated. Other additives investigated in the blends included methylene methane disulfonate (MMDS) and ethylene sulfate (DTD). Ultra-high precision coulometry (UHPC), electrochemical impedance spectroscopy (EIS), gas evolution and long-term cycling experiments were made. The results obtained in this work indicate that MMDS and DTD have good compatibility with PBF-type additives and some of their combinations showed significant improvements in coulombic efficiency, reducing impedance and maintaining good capacity retention during high-voltage cycling compared to PBF-type additives alone.
J. Electrochem. Soc. 2015 volume 162, issue 7, A1186-A1195 , Mar 31, 2015
A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and su... more A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and successfully synthesized. The synthesis is very simple: a pyridine derivative is mixed with boron trifluoride dissolved in diethyl ether to yield a solid crystalline product. The effect of Pyridine-Boron Trifluoride (PBF) and its derivatives have been thoroughly evaluated in Li[Ni1/3Mn1/3Co1/3]O2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells. Evaluation experiments, including high voltage storage, gas production, electrochemical impedance spectroscopy measurements, ultra high precision cycling and long-term cycling were carried out on cells containing the novel additives. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC), prop-1-ene sultone (PES), methylene methane disulfonate (MMDS), tris(-trimethyl-silyl)-phosphite (TTSPi) and triallyl phosphate (TAP). The PBF additives are competitive with all known additives in NMC/graphite cells, which, combined with their low cost and facile synthesis, suggest this series of novel additives will be useful in high-voltage/high-temperature lithium ion battery applications. The PBF additives yield cells which show excellent capacity retention and maintain low impedance during high voltage cycling, in contrast to cells containing VC.
J. Electrochem. Soc. 2015 volume 162, issue 7, A1186-A1195 , Mar 31, 2015
A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and su... more A series of novel electrolyte additives based on Lewis acid/base adducts has been designed and successfully synthesized. The synthesis is very simple: a pyridine derivative is mixed with boron trifluoride dissolved in diethyl ether to yield a solid crystalline product. The effect of Pyridine-Boron Trifluoride (PBF) and its derivatives have been thoroughly evaluated in Li[Ni1/3Mn1/3Co1/3]O2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells. Evaluation experiments, including high voltage storage, gas production, electrochemical impedance spectroscopy measurements, ultra high precision cycling and long-term cycling were carried out on cells containing the novel additives. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC), prop-1-ene sultone (PES), methylene methane disulfonate (MMDS), tris(-trimethyl-silyl)-phosphite (TTSPi) and triallyl phosphate (TAP). The PBF additives are competitive with all known additives in NMC/graphite cells, which, combined with their low cost and facile synthesis, suggest this series of novel additives will be useful in high-voltage/high-temperature lithium ion battery applications. The PBF additives yield cells which show excellent capacity retention and maintain low impedance during high voltage cycling, in contrast to cells containing VC.
Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonat... more Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonates solvents used in lithium ion battery electrolytes. The reaction precipitates have been analyzed by IR-ATR and solution NMR spectroscopy and the evolved gases have been analyzed by GC-MS. The reduction products of ethylene carbonate and propylene carbonate are lithium ethylene dicarbonate and ethylene and lithium propylene dicarbonate and propylene, respectively. The reduction products of diethyl and dimethyl carbonate are lithium ethyl carbonate and ethane and lithium methyl carbonate and methane, respectively. Lithium carbonate is not observed as a reduction product.
Silicon (Si) is a promising candidate for lithium ion battery anodes because of its high theoreti... more Silicon (Si) is a promising candidate for lithium ion battery anodes because of its high theoretical capacity. However, the large volume changes during lithiation/delithiation cycles result in pulverization of Si, leading to rapid fading of capacity. Here, we report a simple fabrication technique that is designed to overcome many of the limitations that deter more widespread adoption of Si based anodes. We confine Si nanoparticles in the oil phase of an oil-in-water emulsion stabilized by carbon black (CB). These CB nanoparticles are both oil- and water-wettable. The hydrophilic/hydrophobic balance for the CB nanoparticles also causes them to form a network in the continuous aqueous phase. Upon drying this emulsion on a current collector, the CB particles located at the surfaces of the emulsion droplets form mesoporous cages that loosely encapsulate the Si particles that were in the oil. The CB particles that were in the aqueous phase form a conducting network connected to the CB cages. The space within the cages allows for Si particle expansion without transmitting stresses to the surrounding carbon network. Half-cell experiments using this Si/CB anode architecture show a specific capacity of ∼1300 mAh/g Si + C and a Coulombic efficiency of 97.4% after 50 cycles. Emulsion-templating is a simple, inexpensive processing strategy that directs Si and conducts CB particles to desired spatial locations for superior performance of anodes in lithium ion batteries.
J. Electrochem. Soc. 2014 volume 161, issue 6, A1001-A1006, May 1, 2014
A comparative investigation of the different lithium salts on formation of the solid electrolyte ... more A comparative investigation of the different lithium salts on formation of the solid electrolyte interface (SEI) on binder free graphite anodes for lithium ion batteries has been conducted. The electrolytes investigated include 1 M LiPF6, LiBF4, LiTFSI, LiFSI, LiDFOB or LiBOB dissolved in ethylene carbonate (EC). The SEI has been investigated via a combination of spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) allows direct observation of the SEI formed from the different electrolytes. Nuclear magnetic resonance (NMR) spectroscopy of D2O extracts are utilized to characterize the soluble species of SEI. XPS and FTIR provide additional elemental and functional group information for the SEI components. The SEI for all electrolytes contains lithium ethylene dicarbonate (LEDC), the primary reduction product of EC. In addition, the SEI for all electrolytes contain LiF except for the SEI generated from the LiBOB electrolyte. The SEI generated in the presence of LiBOB or LiDFOB electrolytes contain multiple oxalate containing species, including lithium oxalate (Li2C2O4), and borates.
J. Phys. Chem. C, 2013, 117 (48), pp 25381–25389, Nov 13, 2013
An investigation of the interrelationship of cycling performance, solution structure, and electro... more An investigation of the interrelationship of cycling performance, solution structure, and electrode surface film structure has been conducted for electrolytes composed of different concentrations of LiPF6 in propylene carbonate (PC) with a binder-free (BF) graphite electrode. Varying the concentration of LiPF6 changes the solution structure, altering the predominant mechanism of electrolyte reduction at the electrode interface. The change in mechanism results in a change in the structure of the solid electrolyte interface (SEI) and the reversible cycling of the cell. At low concentrations of LiPF6 in PC (1.2 M), electrochemical cycling and cyclic voltammetry (CV) of BF graphite electrodes reveal continuous electrolyte reduction and no lithiation/delithiation of the graphite. The solution structure is dominated by solvent-separated ion pairs (Li+(PC)4//PF6–), and the primary reduction product of the electrolyte is lithium propylene dicarbonate (LPDC). At high concentrations of LiPF6 in PC (3.0–3.5 M), electrochemical cycling and CV reveal reversible lithiation/delithiation of the graphite electrode. The solution structure is dominated by contact ion pairs (Li+(PC)3PF6–), and the primary reduction product of the electrolyte is LiF.
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Papers by Mengyun Nie