IN LI-ION BATTERIES

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/423,075, filed November 16, 2016, the entirety of which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under contract number IIP-1621 177 awarded by the National Science Foundation. The government has certain rights in the invention.

TECHNICAL FIELD

[0003] This disclosure relates to energy storage devices such as lithium-ion electrochemical cells and batteries. More specifically, the disclosure relates to improvements to room temperature ionic liquid electrolytes separately and in combination as used in lithium-ion energy storage devices and batteries.

BACKGROUND

[0004] With reference to Figure 1 , previous work by applicants has achieved stable cycling of the silicon + cyclized polyacrylonitrile (Si-cPAN) anode in half-cell configurations and full-cell configurations with "nickel-rich" cathodes (NMC[622] and NMC[81 1 ]). However, incorporation of these materials in a high performing full-cell without electrochemical pre-conditioning and with high rate capability remains a major technical hurdle. A key to the Si/NMC[622] full-cell lies in the early cycle coulombic efficiencies (CEs) of the Si anode. Previous research by applicants on the Si-cPAN system proved that formation of a stable solid-electrolyte interphase (SEI), regarded as the key to a commercially viable silicon anode, results in high half-cell CEs and high capacity retention. However, creating such an SEI in a full-cell system remains a challenge. [0005] Figure 2 illustrates the challenge of this task, demonstrating the relatively low performance of the system shown in Figure 1 when the Si anode is not electrochemically pre-conditioned or pre-lithiated and showing poor stability at higher rates. The cell in Figure 2a shows performance of a cell utilizing "un-matched" electrodes, leading to poor SEI formation over all Si particles; it has been found that the success of this system is heavily dependent on the accurate matching of active material mass loadings in the cathode and anode composites, as well as the design of the Si-cPAN structure based on the ratio of Si mass and cPAN mass, which determines the cPAN coating thickness, electrode porosity, and other parameters. Figure 2b shows performance of a non-preconditioned single-stack pouch full-cell with accurate electrode mass-matching, proving that low-rate cycling stability can indeed be achieved through active material mass matching.

[0006] As alluded to in Figure 2's demonstration of stability at low rates, the stability of the non-preconditioned Si/NMC[622] full-cells is highly sensitive to cycling rate. Higher rates are known to cause "diffusion-induced stress" in the SEI layer, leading to interfacial instability. While this phenomenon is mitigated through SEI formation during pre-conditioning (as shown in Figure 1 ), non-preconditioned cells cannot be run at rates above C/3 (Figure 2a). Innovation is therefore required to enable a Si/NMC[622] full-cell which can stabilize quickly and remain stable at high rates and with thick, high mass loading cathodes for a commercially viable electric vehicle (EV) battery.

[0007] Another key limitation restricts the use of the technology in higher power applications: electrolyte conductivity. Recent research has shown that pyrrolidinium (PYRi„ ⁺ ) and bis(fluorosulfonyl)imide (FSI)-based ionic liquids (ILs or RTI Ls) have higher conductivities compared to their analogs due to lower solution viscosities as well as adequate compatibility with electrode materials such as graphite, UC0O2, and Li(Ni ₁ /3Mn _1/3 Co ₁ /3)02. Despite these advantages, RTI L-based electrolytes are not a magic bullet for enabling the next-generation Li-ion battery.

[0008] RTIL-based electrolyte materials are stigmatized by their ionic conductivities. The RTIL systems typically have Li ⁺ conductivities of about half of state-of-the-art electrolytes. While alternative pack designs can mitigate this issue simply by pairing cells in parallel for increased power, it remains desirable to provide higher rate performance and increased low temperature performance so as to create a universally appealing Li-ion cell. For example, NASA seeks Li-ion technologies capable of maintaining greater than 90% of C/5 room temperature capacities at 0 °C (extravehicular missions) or retaining 80% of C/2 room temperature capacity at -60 °C (lander and rover power sources). Figure 3 illustrates applicants previous work with respect to cathode 60 °C and 0 °C performance in additive-free RTIL-based electrolytes. This system's high temperature stability is owed to the thermal stability of the RTIL electrolyte and interfacial compatibility with high voltage cathode systems, but RTIL electrolyte conductivity hinders low temperature performance.

[0009] Accordingly, the ability to mitigate both SEI formation losses and instabilities and issues arising from RTIL conductivity limitations remains desirable

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Non-limiting and non-exhaustive embodiments of the technology described herein, including preferred embodiments, are described with reference to the following figures, wherein like reference numerals refer to like parts through the various views unless otherwise specified.

[0011] Figure 1 illustrates applicant's μ5ί/ΝΜΟ[622] full-cell technology, cycled in a high purity RTIL-based electrolyte, demonstrating a capacity and energy retention of greater than 80% over 300 cycles (100% depth of discharge) at a C/3 rate. Specific energy normalized to electrode film thickness.

[0012] Figure 2 illustrates the cycling stability of applicant's μ5ί/ΝΜ0622 technology with no electrochemical preconditioning, which is typically required for cells utilizing a silicon anode. Specific capacity/energy normalized to total active material mass.

[0013] Figure 3 illustrates half-cell cycling performance of applicant's high-energy cathode/mRTIL system at temperatures of 60 °C (top) and 0°C (bottom).

[0014] Figure 4 illustrates room temperature full-cell performance of applicant's technology in the mbRTIL electrolyte.

[0015] Figure 5 is SEM micrographs taken subsequent to extended charging of Al corrosion cells at 4.6V vs. Li/Li ⁺ in electrolyte solutions comprised of pure PYR ₁₃ FSI + 1 .2/W LiFSI (a), 1 .2/W LiFSI in PYR13FSI + 10% vol. EC:EMC (b), and 1 .2/W LiFSI in PYR13FSI + 50% vol. EC:EMC (c).

[0016] Figure 6 illustrates rate study of Li(Ni _1/ 3Mn ₁ /3Co ₁ /3)0 ₂ half-cells containing electrolyte solutions comprised of mixtures of PYR ₁₃ FSI + 1 .2/W LiFSI and various volumetric amounts of EC:EMC (1 :2 wt.). Electrochemical cycling was performed at room temperature between 3-4.2V vs. Li/Li ⁺ .

[0017] Figure 7 illustrates cycling data of a μ5ί-οΡΑΝ half-cell containing a fluorinated electrolyte additive showing the rapid CE stabilization achieved through use of a co-salt.

[0018] Figure 8 illustrates EIS of Li/Li symmetric cells containing a RTIL electrolytes with a nitrate salt additive, exhibiting electrolyte conductivity values >6 mS/cm.

DETAILED DESCRIPTION

[0019] Before describing the invention, it is to be understood that the invention is not limited to the details of construction or electrolyte compositions set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.

[0020] Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.

Approach

[0021] One key to the Si anode/high energy cathode full-cell lies in the early cycle CEs of the Si anode and the cathode-electrolyte interface (CEI) of the cathode. The use of electrolyte additives is effective in addressing the thermal stability and safety issues of high energy cathodes through interfacial design while simultaneously providing stable SEI's on the Si-cPAN anode. Electrolyte additives including salts and co-solvents can be employed to increase SEI stability and provide more efficient SEI formation and lower early cycle irreversibility. Via the RTIL-based electrolyte compositions described herein, interfacial stability of the electrode systems is improved. Of significance is the synergistic effect of fluorinated electrolyte additives and others in improving both the safety of the nickel-rich cathode and the performance of the Si anode.

[0022] In order to increase electrolyte conductivity to provide increased rate performance and increased low temperature performance, the integration of low viscosity co-solvents and viscosity reducing salts is employed in electrolytes. Applicant's cathode/RTIL technology maintains greater than 75% of its C/5 room temperature capacity and -63% of its C/2 room temperature capacity in 0°C cycling conditions, about 15% lower than low temperature application goals. Electrolyte conductivity can be boosted using appropriate co-solvents and salts given the relationship between conductivity and viscosity. As viscosity decreases, conductivity increases. It is widely accepted that viscosity is the most important factor determining an IL's ionic conductivity. Addition of low viscosity solvents help to reduce the viscosity of I L according to Walden Rule, which states that the product of the limiting molar conductivity, A _m °, and the solvent's viscosity, η, is constant. ILs have been found to obey this relationship. This supports the claim that adding a co-solvent to ILs can increase ionic conductivity and lithium ion mobility by showing that addition of a co- solvent to PYR ₁₃ FSI + 1 .2M LiFSI leads to significantly increased ionic conductivities.

[0023] One challenge lies in finding appropriate co-solvents and salt additives that do not detract from the favorable electrode-electrolyte interfacial reactions. Thus, described herein are co-solvents with low viscosity, high voltage stability, high thermal stability, and high compatibility with high voltage cathodes and a Si-cPAN anode.

[0024] While this strategy is technically feasible, one difficulty of identifying appropriate co-solvent and salts requires in-depth study and characterization. To demonstrate the validity of this approach, initial trials using RTIL co-solvents have been employed. RTIL co-solvents are an attractive option given the high voltage and thermal stabilities of RTI Ls. The 1 -ethyl-3-methylimidazol-ium bis-fluorosulfonylimide (EMIMFSI) solvent, for instance, possesses low viscosity. Adding just 10% volume EMIMFSI to applicant's mRTIL, creating a "binary" RTIL solvent electrolyte ("mbRTIL"), lowers the mRTI L viscosity from -60 to 70 cP to -45 to 50 cP and increases conductivity to greater than 5 mS cm ^"1 (almost that of conventional organic electrolyte, -8 to 10 mS cm ^"1 ). Figure 4 provides initial results of a full-cell utilizing the mbRTIL electrolyte. The increased electrolyte conductivity decreases ohmic overpotentials in the cell and allows for an increase in initial capacities (+15 mAh/g). However, the lower chemical compatibility of the EMIMFSI solvent with the Si-cPAN anode leads to lower cycling stability.

[0025] This application describes the ability to mitigate both SEI formation losses and instabilities and issues arising from RTIL conductivity limitations through the utilization of electrolyte additives and electrolyte compositions.

Electrochemical and Physical Compatibility and the Limitations of Carbonate Electrolyte Additives

[0026] Electrolyte conductivity can be boosted using appropriate co-solvents given the relationship between conductivity and viscosity. During preliminary screenings of RTIL materials for applications in electrochemical devices, it was found that certain cation-anion combinations lead to the oxidation, or corrosion, of metal components. This oxidation is now known to occur in Li-ion cells containing certain anions when exposed to high voltage cycling conditions, ultimately leading to cell failure as the corrosion prevents the cells from charging properly. In order to develop an RTIL electrolyte capable of use in Li-ion batteries, this problem needs to be understood and mitigated.

[0027] The oxidative decomposition of the aluminum current collector in Li-ion cells containing the FSI ^" or TFSI ^" anions has been studied previously; however, it is desirable to understand this mechanism in cells containing the PYR ₁₃ FSI (1 .2/W LiFSI) electrolyte and develop a strategy to overcome the cell degradation associated with this problem. Adding even a small amount of organic solvent to the electrolyte leads to corrosive behavior when charging cells above 4.2 V vs. Li/Li ⁺ as shown in Figure 5. Initially, RTILs were being investigated as additives to eliminate the concern of flammability associated with organic electrolytes. Organic solvent addition also increases lithium ion mobility in RTILs and can therefore be employed as a strategy to increase RTIL ionic conductivity.

[0028] Adding organic solvent and charging L333 half-cells up to only 4.2 V vs. Li/Li ⁺ (corrosion onset voltage is greater than 4.2 V) results in a corrosion-free increase in rate performance as shown in Figure 6. Despite the slightly lower capacities found using pure RTIL electrolyte, caused by the inherent ohmic resistance of the RTIL solvent, this study proved the potential of FSI-based RTILs as replacements for conventional electrolytes.

Additives for Enhanced SEI Formation

[0029] A number of electrolyte additives capable of forming highly robust SEIs on the Si-cPAN electrode have been identified. In some embodiments, the electrolyte solution includes additives such as fluoroethylene carbonate (FEC) and c(/(2,2,2 trifluoroethyl)carbonate (DFDEC) as potential co-solvent additives, and lithium fluoride (LiF), lithium tetrafluoroborate (LiBF ₄ ), lithium difluoro(oxalato)borate (LiDFOB), and lithium hexafluorophosphate (LiPF ₆ ) as salt additives. While in the past these additives were utilized to increase the cathodic and anodic stability of carbonate electrolytes by fluorine substitution of hydrogen in the carbonate molecules, they can be used to specifically tailor the interfacial compatibility of the Si electrode and cathode during early cycling and under high rate cycling conditions. Such electrolyte additives are expected to be "sacrificial," forming a more stable SEI upon initial charging. Preliminary attempts, using a fluorinated salt additive, demonstrates the high degree of confidence in this approach as shown in Figure 7. The Si-cPAN/RTIL system achieves an outstanding first cycle CE of 90.656%, reaching 99.623% in just 3 cycles. This data suggests the immediate formation of a much more stable interface which is hypothesized to perform better under high rate and high temperature conditions.

Salt Additives for SEI formation

[0030] Salt additives capable of enhancing the SEI of the Si-PAN electrode and the cathode-electrolyte interface (CEI) of the high voltage cathode include lithium fluoride (LiF) borates and derivatives thereof including, but not limited to, lithium tetrafluoroborate (LiBF ₄ ), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalate)borate (LiBOB), lithium bis(malonato)borate (LiBMB), lithium bis(difluoromalonato)borate (LiBDFMB), lithium (malonato oxalato)borate (LiMOB), and lithium (difluoromalonato oxalato)borate (LiDFMOB), dicarbonate and carbonate salts and derivatives thereof including, but not limited to, lithium ethylene dicarbonate (LEDC), phosphate salts and derivatives thereof including, but not limited to, lithium tris(oxalato)phosphate (LiTOP), lithium tris(difluoromalonato)phosphate (LiTDFMP), and lithium hexafluorophosphate (LiPF ₆ ), salts of chelated orthoborates or orthophosphates, imidazolium based salts and derivatives thereof including, but not limited to, lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), sulfonyl-based lithium salts and derivatives thereof including, but not limited to lithium bis trifluoromethanesulfonyl (LiBETI), imide based salts and derivatives thereof including, but not limited to, lithium cyclo-hexafluoropropane-1 , 1 -bis(sulfonyl)imide (LiH-PSI) and lithium bis(trifluoromethane)sulfonimide (LiTFSI), cyclic lithium imide salts, double lithium salts, sultone salts and derivatives thereof including, but not limited to, 1 - propene 1 ,3-sultone, and other sulfanate salts and derivatives thereof comprising the R-SO ₃ ^" functional group including, but not limited to, lithium 1 , 1 ,2,2-tetrafluoro-2- methoxyethansulfonate (LiTFMES), nitrate salts and derivatives thereof including, but limited to, lithium nitrate (LiN0 ₃ ), and lithium 1 , 1 ,2,2-tetrafluoro-ethansulfonate (LiTFES).

[0031] The amount (weight percentage of total electrolyte) of co-salt SEI former is between 0.01 % and 15% with an exemplary weight percentage being between 0.5% and 5%.

Co-Solvent Additives for SEI Formation

[0032] Solvent additives capable of enhancing the SEI of the Si-PAN electrode and the cathode-electrolyte interface (CEI) of the high voltage cathode include ethylene carbonate (EC), derivatives of EC, vinylene carbonate (VC), derivatives of VC, and halogen atom-substituted cyclic carbonates. Specific examples of SEI additives include, but are not limited to, vinylene carbonate (VC), vinylethylene carbonate (VEC), methylene ethylene carbonate (MEC), fluoroethylene carbonate (FEC), chloroethylene carbonate (CEC), methyl(2-oxo-1 ,3-dioxolan-4-yl)methyl) carbonate, tetrahydrofuran, oxolane solvents and derivatives thereof including, but not limited to, 4,4-difluoro-l,3-dioxolan-2-one and 4,5- difluoro-l,3-dioxolan-2-one, propionate solvents and derivatives thereof including, but not limited to, methyl tetrafluoro-2-(methoxy)propionate (MTFMP), oxathilane solvents and derivatives thereof including, but not limited to, 1 ,3,2-dioxathilane-2,2-dioxide (DTD), succinic and methyl succinic anhydride, maleic and methyl maleic anhydride, disubstituted maleic anhydride containing fluorine substitutions, theophene solvents and derivatives thereof including, but not limited to, 3-hexylthiophene, and 1 -fluoropropane-2-one.

[0033] The amount (weight percentage of total electrolyte) of co-solvent SEI former is between 0.01 % and 20% with an exemplary weight percentage being between 0.5% and 5%.

Supplemental Lithium Sources

[0034] Supplemental lithium sources capable of offsetting early cycling irreversibility include lithium nitride (Li ₃ N). These additives can allow for more advantageous (lower) N/P ratios and increased reversible capacity by providing a secondary lithium source to compensate for inefficiencies during the formation cycles.

[0035] The amount (weight percentage of total electrolyte) of supplemental lithium source is between 0.01 % to 5%, with an exemplary weight percentage being between 0.05% and 2%.

Additives for Enhanced Electrolyte Conductivity

Co-Salt Additives for Enhanced Conductivity

[0036] Salt additives capable of enhancing the conductivity of Li-ion battery electrolytes, especially those including ionic liquid solvents, include nitrate salts and derivatives thereof including, but not limited to lithium Nitrate (LiN03) and potassium Nitrate (KN03), lithium 3,9-diallyl-3,9-difluoro-2,4,8, 10-tetraoxo-1 ,5,7, 1 1 -tetraoxa-6- boraspiro[5,5]undecan-6-uide, and sulfate salts and derivatives thereof including, but not limited, to lithium sulfate (LJ2S04).

[0037] The amount (weight percentage of total electrolyte) of co-salt additive for conductivity enhancement is between 0.01 % to 75%, with an exemplary weight percentage being between 0.01 % and 20%, and a preferred weight percentage being between 0.05% and 5%. Co-Solvent Additives for Enhanced Conductivity

[0038] Co-solvent additives capable of reducing electrolyte solution viscosity and increasing electrolyte conductivity include a range of solvent classes:

[0039] Carbonates: useful carbonate solvents include cyclic carbonates, such as propylene carbonate (PC) and butylene carbonate, and linear carbonates, such as dimethyl carbonate (DMC), diethyl carbonate, dipropyl carbonate, dibutyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. Useful carboxylate solvents include, but are not limited to: methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate. Other carbonates include Di-Methoxyethane (DME) ethylene carbonate (EC), vinylene carbonate (VC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), and methyl propyl carbonate (MPC).

[0040] Phosphates, phosphonates, and phosphites: useful phosphate solvents include, but are not limited to, allyl phosphate, trimethylphosphate, triethyl phosphate, tris(2-chloroethyl) phosphate, propyl dimethyl phosphate, dipropyl methyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, tripropyl phosphate, triisopropyl phosphate, tributyl phosphate, trihexyl phosphate, tritolyl phosphate, methyl ethylene phosphate, ethyl ethylene phosphate, alkyl phosphonates including trimethylphosphonate, and propyl dimethylphosphonate, and aromatic phosphonates, such as phenyl dimethylphosphonate and triphenyl phosphate. Exemplary amounts (weight percentage of total electrolyte) of phosphorus containing additives for conductivity enhancement are between 1 % and 4%. Phosphites include, but are not limited to, tris(2,2,2)-trifluorethyl phosphite, tris(trialkylsilyl)phosphites, tris(trimethylsilyl)phosphite, tris(triethylsilyl)phosphite, and tris(tripropylsilyl)phosphite.

[0041] Borates: useful borate solvents include, but are not limited to, tri-ethyl Borate (TEB), tris(trialkylsilyl)borates tris(trimethylsilyl)borate, tris(triethylsilyl)borate, and tris(tripropylsilyl)borate.

[0042] Nitriles: useful nitrile solvents include, but are not limited to, acetonitrile, fluoroacetonitrile (FAN), adiponitrile, propionitrile, butyronitrile and dinitriles (CN[CH ₂ ] _n CN) with various alkane chain lengths (n = 1 -8). [0043] Sulfones: useful sulfone solvents include, but are not limited to, fully fluorinated sulfones, such as di(thfluoromethyl) sulfone, di(pentafluoroethyl) sulfone, thfluoromethyl pentafluoroethyl sulfone, thfluoromethyl nonafluorobutyl sulfone, and pentafluoroethyl nonafluorobutyl sulfone. non-fluorinated sulfones such as dimethyl sulfone, ethyl methyl sulfone, and ethyl methoxythyl sulfone, and partially fluorinated sulfones such as methyl trifluoromethyl sulfone, ethyl thfluoromethyl sulfone, methyl pentafluoroethyl sulfone, and ethyl pentafluoroethyl sulfone.

[0044] Ionic liquid: useful ionic liquids include, but are not limited to, ionic liquids containing the N-propyl-N-methylpiperidinium cation and other piperidinium cations, ionic liquids containing the N-methyl-N-propyl pyrrolidinium cation and other pyrrolidinium cations, bis(oxalate)borate (BOB) anion based ionic liquids including N- cyanoethyl-N-methylprrrolidinium BOB, 1 -methyl-1 -(2-methylsulfoxy)ethyl)- pyrrolidinium BOB, and l-methyl-l-((l,3,2- dioxathiolan-2-oxide-4- yl)methyl)pyrrolidinium BOB, tris(pentafluoroethyl)trifluorophosphate (FAP) anion based ionic liquids, such as N-allyl-N-methylpyrrrolidinium FAP, N-(oxiran-2- ylmethyl)N-methylpyrrolidinium FAP, and N-(prop-2-inyl)N-methylpyrrolidinium FAP, bis(trifluoromethanesulfonyl)imide (TFSI) anion-based ionic liquids, such as N-propyl- N-methylpyrrolidinium TFSI, 1 ,2-dimethyl-3-propylimidazolium TFSI, l-octyl-3-methyl- imidazolium TFSI, and 1 -butyl- methylpyrrolidinium TFSI, bis(fluorosulfonyl)imide (FSI) anion-based ionic liquids, such as N-Butyl-N-methylmorpholinium FSI and N-propyl-N- methylpiperidinium FSI, and ionic liquids including the l-ethyl-3-methylimidazolium cation and other imidazolium cations including l-ethyl-3-methylimidazolium tetrafluoroborate.

[0045] Polydimethylsiloxane (PDMS) oils: PDMS polymers are liquids at room temperature, and short chain variations afford low-viscosity and chemically stable co- solvents. They are not miscible with the RTIL in notable quantities but do seem to afford some reduction in viscosity and increase in conductivity. The ionic liquid electrolytes have poor miscibility with PDMS and with many separators. Using a surfactant can aid by improving separator wettability and with enhancing co-solvent miscibility. Surfactants include lithium dodecyl sulfate and other sulfates, nitrates, phosphates, and borates. Most surfactants are potassium and sodium salts. Other surfactants can be prepared by neutralization of the acid with lithium hydroxide. [0046] Glymes: Glycol ethers, or glymes, are a class of increasingly common "green solvents" due to their low vapor pressure, high boiling point, high thermal stability, and high (electro)chemical stability. Glyme solvents, along with diglyme and tetraglyme (diethyl- and tetraethyl-, respectively) may be effective co-solvents that can decrease the electrolyte conductivity while retaining the high SEI stability offered by the IL electrolyte. Useful glymes include triethyl glycol dimethyl ether (triglyme or "G3"), tetraglyme (G4), and solvents of the tyle Li(glyme)]X with different anions (X: [N(S0 ₂ C ₂ F5)2] or [BETI], [N(S0 ₂ CF ₃ ) ₂ ] or [TFSA], [CF3SO3] or [OTf], BF ₄ , N0 ₃ ).

[0047] Sulfur-containing solvents: Useful sulfur-containing solvents include, but are not limited to, sulfites, sulfates, sulfoxides, sulfonates, thiophenes, thiazoles, thietanes, thietes, thiolanes, thiazolidines, thiazines, sultones, and sulfones. Various degrees of fluorine substitution can be introduced up to and including the fully perfluorinated compunds. Specific examples of sulfur-containing linear and cyclic compounds include ethylene sulfite, ethylene sulfate, thiophene, benzothiophene, benzo[c]thiophene, thiazole, dithiazole, isothiazole, thietane, thiete, dithietane, dithiete, thiolane, dithiolane, thiazolidine, isothiazolidine, thiadiazole, thiane, thiopyran, thiomorpholine, thiazine, dithiane, dithiine, thiepane, thiepine, thiazepine, prop-l-ene- 1,3-sultone; propane- 1 , 3 -sultone, butane- 1 ,4-sultone, 3 -hydroxy- 1 - phenylpropanesulfonic acid 1 ,3 -sultone; 4-hydroxy-l-phenylbutanesulfonic acid 1 ,4- sultone; 4-hydroxy-l-methylbutanesulfonic acid 1 ,4 sultone, 3-hydroxy-3- methylpropanesulfonic acid 1 ,4-sultone, 4-hydroxy-4-methylbutanesulfonic acid 1 ,4- sultone. Sulfones having the formula R-S-C-R ₂ where R and R ₂ are independently selected from the group consisting of substituted or unsubstituted, saturated or unsaturated Ci to C ₂₀ alkyl or aralkyl groups. Other sulfur-containing solvents include propane-l,3-sultone, butane-1 ,4- sultone and prop-l-ene-l,3-sultone.

[0048] Lactones: useful lactone solvents include, but are not limited to, butyrolactone, 2-methyl-y-butyrolactone, 3-methyl-y-butyrolactone, 4-methyl-y- butyrolactone, β-propiolactone, and δ-valerolactone.

[0049] The amount (weight percentage of total electrolyte) of co-solvent additive for conductivity enhancement is between 0.01 % to 75%, with an exemplary weight percentage being between 0.5% and 50% and a preferred weight percentage being between 25% and 50%. Exemplary Cell Design

[0050] The anode is an electrode into which positive electric charge (in the form of lithium cations) flows during normal operation (discharging) when incorporated into an energy storage and conversion device, such as a rechargeable lithium-ion battery. In some embodiments, the anode includes one or more active material particle enclosed by a membrane permeable to lithium ions. The active material particles are a source of electrons when the anode is incorporated into an electrochemical cell.

[0051] In some embodiments, the anode includes silicon. In some embodiments, the anode includes micron-sized silicon (μβϊ). The anode allows for reversible cycling of the micron-sized silicon particles. In some embodiments, each active material particle has a diameter in the range of from about one to about fifty micrometers. In some embodiments, each active material particle has a diameter in a range of between about 500 nanometers and about one micrometer, one and about five micrometers, between about one and about ten micrometers, between about one and about twenty micrometers, between about ten and about twenty micrometers, between about ten and about fifty micrometers, or between about twenty and about fifty micrometers. In some embodiments, the active material particles comprising the anode composite may include a mixture of particle sizes ranging from about 500 nanometers to about fifty micrometers.

[0052] In some embodiments, the membrane is a flexible structure enclosing each of the one or more active material particles. In some embodiments, the membrane may enclose one or multiple active material particles. In some embodiments, the membrane includes a conductive material. In some embodiments, the conductive material includes a mechanically resilient polymer (a polymer having the ability to expand and contract with minimal mechanical failure), such as polyacrylonitrile (PAN). In some embodiments, the polymer is self-cyclizing with heat treatment. In some embodiments, the polymer comprises cyclized polyacrylonitrile (cPAN).

[0053] The cathode is an electrode by which electrons enter the energy storage device during discharging. The cathode is not limited to being formed from a particular material. In some embodiments, the cathode includes a transition metal oxide material, such as a nickel-rich oxide material having the formula Li(Ni _x MnyCo _z R _w )0 ₂ (x+y+z+w=1 , x>1 /3, R = Aluminum or other metal). In some embodiments, the nickel- rich oxide material includes Li(Ni ₀ .6Mno _.2 Coo _.2 )0 ₂ ("NMC622" or "NMC[622]"), Li(Nio. ₈ Mno. ₁ Coo. ₁ )0 ₂ ("NMC81 1 " or "NMC[81 1 ]"), or Li(Nio. ₈ Coo.i 5Alo.o5)0 ₂ ("NCA").

[0054] By utilizing the imide-based PYR13FSI (1 .2M LiFSI) electrolyte, along with electrolyte additives, the nickel-rich oxide cathode material and silicon anode may be successfully stabilized. In addition, cycling between 2.5-4.5 V vs. Li/Li ⁺ in this imide- based electrolyte allows for 100% capacity retention over 100+ cycles.

[0055] Further details regarding exemplary cell design suitable for use with the electrolyte described herein is set forth in International Published Patent Application No. WO 2016/123396, the entirety of which is hereby incorporated by reference.

[0056] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited as by the appended claims.