Solid Electrolyte Interphase Stabilization Path to Lithium Metal Plating-Free High-Energy Lithium-Ion Battery Under Subzero-Temperature

When we took a look at the particle morphology of the surface of anodes and cathodes after 100 cycles under subzero-temperature, more pronounced changes were found on graphite anodes than on cathodes. The millimeter-scale photo (Fig. 3b) of graphite anode cycled in commercial electrolyte exhibits somewhat non-uniform and rough surface with clearly distinguished bright and dark parts, compared to smooth surface of black pristine anode (Fig. 3a). Correlating to the abnormal cycling behavior above mentioned due to lithium metal plating (Fig. 2e), this visual non-uniformity confirms the presence of lithium metal plating. ²⁴ Its SEM image (Fig. 3b') find a severely rough surface and cracks of graphite micro-particles. This morphology change can cause enlarged anode-electrolyte interfacial area and thus intensified reductive decomposition of electrolyte, detrimentally affecting the performance (Figs. 2a, 2e, 2f) and rising the impedance (Fig. 2a'). Upon the use of FEC additive (Fig. 3c), the photo of cycled anode shows improved uniformity, but clearly yellow and shiny lithium metal plated during subzero-temperature cycling. FEC additive plays a role in reducing particle morphology change in SEM image (Fig. 3c'), despite still the occurrence of unwanted lithium metal plating. This is associated with reduced impedance and its change (Fig. 2b'). By replacing the electrolyte with nonflammable one, the surface (Fig. 3d) of cycled anode becomes smooth like pristine, indicating no lithium metal plating. The image (Fig. 3d') finds a coverage of a graphite particle with newly produced surface species with tiny particles. The use of FEC additive also permits no lithium metal plating (Fig. 3e) and well-preserved particle morphology (Fig. 3e'). Newly produced surface species probably by electrolyte decomposition are clearly present. The stable cycling performance (Figs. 2e–2f) of nonflammable electrolyte systems under −10 °C is ascribed to the inhibition of lithium metal plating at graphite anode, significantly improved surface passivation with newly produced surface species and the preservation of particle morphology of graphite.

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Raman spectra in Fig. 4a provide information on structural changes of graphite with cycling under −10 °C. Pristine (Fig. 4a–i) shows a prominent peak of G-band at 1580 cm⁻¹, attributed to E_2g mode due to the in-plane vibrations of carbon atoms, and a trace of D-band at 1340 cm⁻¹ corresponding to A_1g mode due to the break of symmetry occurring at the edge planes and defects in the graphene sheets and structural disordering. ^11,25–27 We utilize the intensity ratio of D-band (I_D ) to G-band (I_G ) as a measure of structural disorder. ^11,25–27 The lower the ratio, the greater structural preservation. The I_D /I_G ratio (Fig. 4b) increases from pristine (0.096) to cycled anodes, due to structural disordering. The use of FEC additive in any electrolyte reduces the ratio in common, indicating improved structural preservation. The most structural disorder (i.e., the highest ratio) occurs by commercial electrolyte (0.350), whereas the least (i.e., the lowest ratio) by nonflammable electrolyte with FEC additive (0.168). This point emphasizes that the replacement of electrolyte to nonflammable one leads to significantly improved structural stability of graphite. It is surprising that electrolyte affects to the structure of graphite. In addition, the presence of a new band at 1617 cm⁻¹ of D'-band (Fig. 4a) on all cycled anodes is an indication of surface defects. ^11,28,29 As predicted, while D'-band is the strongest in commercial electrolyte, it is the weakest in nonflammable electrolyte with FEC additive. Relative strength of the D'-band is generally in the same tendency to the change in I_D /I_G . In commercial electrolyte, the occurrence of non-uniform lithium metal plating (Fig. 3b) along with interfacial reaction behavior at graphite anode might cause the structural disorder of graphite probably first at the local surface. Then the structural disorder is propagated to larger area of surface to near surface toward bulk during repeated cycling. The absence of lithium metal plating and the stabilized interface when using nonflammable electrolyte with FEC additive should be correlated to largely improved structural stability.

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ATR FTIR spectroscopy is a surface sensitive tool to probe qualitatively and quantitatively organic SEI species at the 500 μm depth from the electrode surface and on the electrode area of ≈12 mm², which covers overall anode-electrolyte SEI formation behavior. Note that after washing with DMC, just stable solid precipitates remain at anode surface. Figure 5 exhibits IR spectra of pristine and cycled graphite anodes obtained from full-cells with nonflammable electrolyte without and with FEC additive under 4.45 V and −10 °C. Since unwanted lithium plating occurred at graphite anode in commercial electrolyte systems (Figs. 2a, 2b, Figs. 3b, 3c), we did not measure IR spectra for those to avoid the damage of a Ge IR optic by the reactive lithium metal. The IR spectra of cycled graphite anodes (Fig. 5a–iv, v) show prominent peaks at 2970–2910 cm⁻¹ and 1460–1413 cm⁻¹ attributed to methyl (CH₃–) and methylene (–CH₂–) groups of alkyl functionalities that come from newly formed surface species, in addition to carboxylic acid functionalities from PAA and CMC binders of pristine graphite (Fig. 5a–i). ^4,7,11,21,22 Other commonly observed are peaks at 1740–1720 cm⁻¹ due to ν_sym(C=O) of ester (RCO₂R'; R,R' = alkyl group), which are confirmed by the peaks at 1270 cm⁻¹ and 1160 cm⁻¹ by ν(C−O−C)_asym and ν(C−O−C)_sym. Also commonly observed are peaks at 1650–1620 cm⁻¹ of ν_sym(C=O) of alkyl carbonate salt ROCO₂ ⁻Mⁿ⁺ (M = Li/Ni/Co/Mn) and those at 1570 cm⁻¹ of ν_sym(C=O) of carboxylate salt RCO₂ ⁻Mⁿ⁺. ^{7,21,23,24,30–36} Peaks at 1270–1250, and 1084 cm⁻¹ are due to ν(C-F) from –CF₃ and –CF₂- functionalities that are of FEMC and DFDEC decomposition products. ^7,32,37 A peak near 1470 cm⁻¹ is due to ν(CO₃)_sym of Li₂CO₃, along with δ(CO₃) at 850 cm⁻¹, and peaks at 1320 and 1040 cm⁻¹ of ν(P=O), ν(P−O−C)_asym are of organic phosphorus fluorides (O=PF_3–y(OR)_y). ^7,24,32–36 What is distinguished upon the use of FEC additive is relatively lower concentration of Li₂CO₃ and the presence of a tiny new feature at 1780 cm⁻¹, attributed to poly(FEC) coming from FEC (Fig. 5a–v). ^11,33,36,38

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The SEI species are cross-checked with C 1s, F 1s and P 2p XPS spectra. In the C 1s spectra, after cycling (Fig. 5c–ii, iii, iv, v), new peaks of C–O at 286.8 eV, C=O at 288.3 eV and CO₃ at 290 eV of organic compounds such as ester, alkyl carbonate salt and carboxylate salt are commonly observed, in addition to the major peak at 284.5 eV due to C–C/C–H coming from carbon black and the peaks of CMC and SBR binders of pristine graphite anode (Figs. 5c–i), which are consistent with O 1s spectra (Fig. S2a). ^{4,7,11,24,39,40} Upon the use of nonflammable electrolyte without and with FEC additive (Fig. 5c–iv, v), a trace of peak of –CF₃, due to FEMC and DFDEC decomposition products, is observed near 292 eV, consistent with its presence at 688–689 eV in the F 1s spectra (Fig. 5c). ^7,41 In the F 1s spectra, all cycled graphite anodes (Fig. 5c–ii, iii, iv, v) exhibit a dominant peak of LiF at 685 eV and a shoulder of Li_xPO_yF_z, Li_xPF_y and/or organic phosphorus fluoride (O = PF_3-y(OR)_y) of LiPF₆-derived species at 687.2 eV, as confirmed in the Li 1s (Fig. S2b) and P 2p spectra (Fig. 5c). ^{7,19,21,23,24,31–36,39,40} In particular, those features are significant in nonflammable electrolyte systems (Fig. 5c–iv, v), indicating the formation of F-based inorganics-rich SEI layer. The F 1 s spectrum of commercial electrolyte with FEC additive (Fig. 5c–iii) shows a strong peak of MnF₂, NiF₂ and CoF₂ near 686 eV relative to the peak of LiF. ^{25,27,30,32–35} The presence of such deposits of metal species at graphite anode implies the occurrence of metal-dissolution from NCM811 cathode, which is discussed in later section.

The resultant data demonstrate the effects of nonflammable electrolyte and FEC additive in changing the SEI composition and concentration on graphite anode in full-cell under −10 °C; nonflammable electrolyte with FEC additive induces to form both organics-rich and inorganics-rich SEI layer, compared to that of commercial electrolyte systems. Superior cycling performance of full-cells in nonflammable electrolyte systems should be correlated to this effective surface passivation with the SEI layer. If metal-dissolution from cathode occurred in commercial electrolyte systems under −10 °C, that might another serious origin of performance failure in addition to lithium metal plating.

Electrolyte-induced bulk structural change in the NCM811 cathode with cycling under 4.45 V and −10 °C was examined using ex-situ powder XRD analysis as shown in Fig. 6. XRD pattern of pristine NCM811 cathode (Fig. S4–i) was identified as crystallized in hexagonal (Rm) layered structure with the a and c lattice parameters of 2.8688 and 14.1989 Å, respectively, and the c/a ratio of 4.9495 and unit cell volume of 101.1951 ų (Fig. 6d). ^24,42–44 After cycling in commercial electrolyte (Figs. 6a, 6b, 6c–ii), the peaks of (003), (101) and (104) reflections are largely shifted to higher 2θ region from that of pristine, causing a substantial decrease both in the a and c parameters (Fig. 6d). Such large peaks shift is attributed to the phase transformation from original hexagonal H1 phase to H3 as marked in the figures. ^45,46 During charging (Li⁺-deintercalation), the H1 phase of NCM811 transforms to monoclinic phase (M) and hexagonal H2 phase and then another hexagonal H3 phase. ^45,46 The presence of H3 phase, moreover, in the absence of H1 indicates a full phase transformation to H3 and severe structural disorder, despite cycling was ended as discharged. Note that the H1-to-H2 and H1-to-H2-to-H3 phase transformation during cycling is known to lead to a drastic lattice volume change, internal stresses and cracks along the grain boundaries. ⁴⁷ This phase transformation and peaks shift to higher 2θ region are unusual phenomena that are observed only under subzero-temperature, which is distinguished from the commonly accepted tendency that the cycling of NCM active material in both half-cell and full-cell at room and elevated temperatures brings about the shift of (003) reflection to lower 2θ region. ⁴⁸ As a result, with cycling under −10 °C and high voltage 4.45 V, the unit cell is subject to the isotropic abrupt contraction in both c- and a-directions (Figs. 6d, 6e) and never returns to its respective initial state. The crystal structure seems to freeze due to subzero-temperature condition and is not mechanically flexible probably. Peak broadening and weakening support structural disorder and the loss of crystallinity. The unit cell volume thus largely decreases by 5.3% (Fig. 6d), which is a notable difference from the cell volume reduction by just 0.4% occurred in nonflammable electrolyte system by anisotropic contraction only in a-direction. The formation of cracks in the primary NCM811 particles in commercial electrolyte (Fig. S3b) and a drastic performance failure just after 3 cycles (Fig. 2e) are in part ascribed to the significant and irreversible structural disorder and large volume change. The cracks generate enlarged cathode-electrolyte interfacial area and permits electrolyte penetration through the crannies, which accelerate the performance fade (Figs. 2a, 2e, 2f) and impedance rise (Fig. 2a'). Upon the use of FEC additive to commercial electrolyte (Figs. 6a, 6b, 6c–iii), the peaks of (003), (101) and (104) reflections are less shifted to higher 2θ region than those of commercial electrolyte only (Figs. 6a, 6b, 6c–ii). This is attributed to the structural transformation from H1 to H2 phase and still the occurrence of isotropic contraction in both c- and a-directions (Fig. 6e). ^45,46 This is another unusual phenomenon observed only under subzero-temperature, which is distinguished from the co-existence of H2 phase with H1 phase on cycled NCM811 cathode in commercial electrolyte with 3 wt% FEC additive at room and elevated temperatures. ⁴⁸ Relatively smaller unit cell volume reduction by 4.1% (Fig. 6d) is made upon the use of FEC additive. On the other hand, with cycling in nonflammable electrolytes systems (Fig. 6a–iv, v), the peak of (003) reflection is in common shifted to lower 2θ region from that of pristine, indicating lattice expansion in c-direction (Figs. 6d, 6e) while maintaining the original H1 phase of NCM811. ⁴⁴ By the slight contraction in a-direction (Figs. 6b, 6c, 6d), unit cell volume increases just 0.12% from that of pristine. This improved structural maintenance should be correlated to reaction reversibility and improved performance. By using FEC additive to nonflammable electrolyte, the significantly improved structural stability is achieved; unit cell volume is maintained close to that of pristine (Fig. 6d). The ratio of peak intensity, R' = [I₍₁₀₂₎ + I₍₀₀₆₎]/I₍₁₀₁₎], was utilized as a relative measure of cation mixing and structural order by limiting to the H1 phase of NCM811 cathodes cycled in nonflammable electrolytes. ^49,50 The cathodes cycled with commercial electrolyte systems that undergo an irreversible phase transition to H2 or H3 phase are thus excluded. The closer the R' value to that of pristine, the higher the structural stability. The R' (Fig. 6d) of pristine is 0.574, representing a good structural order referred to literatures. ^49,50 After cycling with nonflammable electrolyte, R' slightly increases to 0.691 by structural change. With FEC additive, R' is near the value of pristine, ensuring higher structural stability, supporting the smallest unit cell volume change. These are interesting results to demonstrate how electrolyte composition influences the stability of bulk crystal structure of NCM811 active material by high-voltage cycling under −10 °C, and how the selection strategies of electrolyte and additive are critically crucial in achieving a high structural preservation and stable cycling performance.

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We investigated how electrolyte composition influences the SEI composition of NCM811 cathodes of full-cells with cycling under 4.45 V and −10 °C. Figure 6f compares the IR spectra of pristine and cycled NCM811 cathodes obtained after DMC washing. Pristine (Fig. 6f–i) shows prominent absorbance peaks at 1400, 1170 and 1072 cm⁻¹, characteristic of PVdF binder. ^{7,8,21,23,24,30,32,34,36,51} The spectra of all cycled cathodes (Fig. 6f–ii ∼ v) exhibit in common the peaks at 2972–2908 and 1469–1411 cm⁻¹ due to methyl (CH₃–) and methylene (–CH₂–) groups of alkyl functionality of PVdF binder and newly produced surface species. ^24,30,47,48 Relatively stronger peak absorbance of alkyl functionality on the nonflammable electrolyte with FEC additive (Fig. 6f–v) is related to the presence of organic compounds with longer alkyl chain or at higher concentration than those of other electrolytes. Newly observed in common are the peaks at 1780–1770 and 1730 cm⁻¹, of ν_sym(C=O) from various ester (−CO₂R) compounds, which are confirmed with the peaks at 1278 and 1160 cm⁻¹ by ν(C−O−C)_asym and ν(C−O−C)_sym. ^{7,21,23,24,30,32,34,36,50} Also commonly observed are new peaks at 1620 cm⁻¹ and 1570 cm⁻¹ of ν_sym(C=O) of alkyl carbonate salt ROCO₂ ⁻Mⁿ⁺ (M = Ni/Co/Mn/Li) and carboxylate salt RCO₂ ⁻Mⁿ⁺, respectively. ^{7,21,23,24,30,32,34,36,51} On the cycled cathode in commercial electrolyte (Fig. 6f–ii), a tiny peak near 1470 cm⁻¹ is due to ν(CO₃)_sym of Li₂CO₃, which is a well-known EC decomposition product and observed in the Li 1 s XPS spectra (Fig. 7d). ^7,32,50–52 Also newly observed are weak peaks at 1310–1010 cm⁻¹ of ν(P=O) and ν(P–O–C)_asym of POF_3−y(OR)_y and the ones at 918–780 cm⁻¹ of ν(P–F) coming from PF-containing species like Li_xPF_y, and Li_xPO_yF_z, which are typical decomposition products of LiPF₆:EC solvate. ²⁴ With FEC additive to commercial electrolyte (Fig. 6f–iii) or nonflammable electrolyte (Fig. 6f–v), a new tiny peak is observed at 1780 cm⁻¹, attributed to poly(FEC) formed by FEC polymerization. ^11,32,33,38 While cycled cathodes with nonflammable electrolyte without (Fig. 6f-iv) and with FEC additive (Fig. 6f–v) exhibit slightly stronger peaks of ester along with stronger peaks of alkyl groups than those of commercial electrolyte systems (Fig. 6f–ii, iii), implying higher concentration, alkyl carbonate salt and carboxylate salt are notably at lower concentration. New peaks at 1250, 1110 and 1084 cm⁻¹, attributed to ν(C–F) of –CF₃ and –CF₂– functionalities, of nonflammable electrolytes might be of the decomposition products of FEMC and DFDEC. ^{24,34,36,51–53} The IR analysis results reveal that the surface of cycled NCM811 cathode in nonflammable electrolyte without and with FEC additive are covered with plenty of esters and CF-containing species, perhaps by oxidative decomposition of LiPF₆:PC solvate, and FEMC and DFDEC solvents.

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Figure 7 presents XPS spectral comparison of the surface elements of NCM811 cathodes of full-cells before and after cycling under 4.45 V and −10 °C. The formation of soluble Ni²⁺–O and Mn²⁺–O at cathode surface has been established as a clear indication of metal-dissolution and oxygen gas release from NCM cathodes for maintaining a charge-neutrality and as surface structural degradation and reconstruction from hexagonal layered to disordered phase. ^7,24,35 Earlier works showed that such surface structural change is propagated toward bulk, causing particle cracking and disintegration and impedance rise. ^7,24,35 The lower the fraction of surface Ni²⁺–O and Mn²⁺–O on cycled cathode, the greater the maintenance of surface structure and morphology. ^{7,23,24,32–36} In the Ni 2p spectra of cycled cathodes (Fig. 7a), the asymmetric shape of 2p_3/2 peak at 855 eV implies the mixed valence of Ni atom. Surface Ni valance was determined by curve-fitting on the 2p_3/2 peak, which is separated from a shakeup satellite at 861.4 eV, employing multiple-splitting patterns derived from the standard compounds of NiO, Ni₂O₃ and NiF₂ and at 854.2, 856.1 and 857.8 eV, respectively. ^{7,24,34,35,39–41} Fitting results are summarized in Fig. 7e. The surface of pristine (Figs. 7a–i and 7e) consists of 41% Ni²⁺–O and 59% Ni³⁺–O. After cycling in commercial electrolyte systems (Figs. 7a–ii,iii and 7e), the fraction of total surface Ni²⁺ (as Ni²⁺–O and Ni²⁺–F) largely increases to 75% and 64% without and with FEC additive, respectively. An increase in the fraction of surface Ni²⁺ with cycling is generally interpreted as the occurrence of electrochemical oxidation of electrolyte during charging (Li⁺-deintercalation), consequent electron transfer from electrolyte to surface Ni⁴⁺–O that forms during charging for charge-neutrality, and thus instant reduction of reactive surface Ni⁴⁺ to Ni³⁺ to Ni²⁺. ^7,28,37 In the course of such surface reduction, some Ni²⁺ uptakes F⁻ ion from electrolyte and forms a stable NiF₂ as observed in the Ni spectra, ^{7,23,24,34–36,39–41,52} which is consistent with its presence in F 1 s spectra (Fig. 7c). However, such problematic soluble Ni²⁺–O formation is somewhat suppressed to 28% and 32% upon the use of nonflammable electrolyte without (Figs. 7a–iv and 7e) and with FEC additive (Figs. 7a–v and 7e), respectively. Cycling in nonflammable electrolyte system is ended as enriched surface state with Ni³⁺–O (Fig. 7e) that is closer to pristine, emphasizing the surface stabilization and passivation effect by nonflammable electrolyte. However, the concentration of LiF (Fig. 7d–iv, v), and CoF₂ as well as NiF₂ (Figs. 7c–iv,v and 7e) largely increase. ^7,24,54 Upon the use of FEC additive, both commercial electrolyte and nonflammable electrolyte give in common the reduced fraction of NiF₂ (Fig. 7e) together with LiF (Figs. 7c, 7d) and CoF₂ (Fig. 7c), despite the expected role of FEC as an F-source. This in turn implies FEMC and DFDEC to be the F-sources but the dominant role of FEC additive to be a SEI forming agent on both NCM811 cathode and graphite anode.

Surface Mn valance was also determined by curve-fitting on the Mn 2p_3/2 peak, employing multiple-splitting patterns derived from the standard compounds of MnO, Mn₂O₃ and MnO₂ at 641.0, 641.7 and 642.7 eV, respectively. ^{7,23,24,33–36,39–41} Fitting results (Fig. 7f) reveal that 100% Mn⁴⁺–O of pristine surface is no longer maintained after cycling with any electrolyte. With commercial electrolyte (Figs. 7b–ii and 7f), 48% Mn²⁺–O forms at the expense of Mn⁴⁺–O. The electrochemical reduction of Mn⁴⁺–O to Mn³⁺–O followed by disproportionation of Mn³⁺–O to a half Mn⁴⁺–O and a half Mn²⁺–O might also occur in the course of electrolyte oxidation and electron transfer to surface Mn⁴⁺–O of NCM811 during charging. ^{7,23,24,33–36,41} A trace of MnF₂ at 655.7 eV, confirmed with the peak at 685.5 eV in F 1 s spectrum (Fig. 7c–ii), indicates a possible interfacial reaction between Mn²⁺ and F⁻ coming from electrolyte. ^{7,23,24,33,36,55} Upon the use of FEC additive (Figs. 7b–iii and 7f), three mixed valent Mn atoms of Mn⁴⁺–O, Mn³⁺–O and Mn²⁺–O coexist. The presence of Mn³⁺–O together with reduced fraction (22%) of Mn²⁺–O indicates suppressed MnO-dissolution. ³⁰ The use of FEC additive (Figs. 7c–iii) brings about unchanged weak peak of MnF₂ in F 1s spectrum, supporting the dominant role of FEC as the SEI forming agent for NCM811 cathode rather than a F-source for forming metal fluorides. On the other hand, nonflammable electrolyte (Figs. 7b–iv and 7f) drives greater maintenance of Mn⁴⁺–O to 78%, despite the formation of Mn²⁺–O at 22%. The use of FEC additive (Figs. 7b–v and 7f) leads to the absence of Mn²⁺–O, but just the presence of 55% Mn⁴⁺–O and 45% Mn³⁺–O. Cathodes display a shoulder at 688–689 eV due to C–F bond of FEMC and DFDEC-derived CF-containing species and a peak at 687.4 eV of LiPF₆-derived POF_3-y(OR)_y/Li_xPO_yF_z species, consistent with P 2p spectra (Fig. S5c). ^23,24,36

XPS analysis results reveal that nonflammable electrolyte induces to reduces the problematic surface Ni²⁺–O and Mn²⁺–O, which inhibits metal-dissolution and surface structural degradation of NCM811 under 4.45 V and −10 °C. Moreover, the use of FEC additive eliminates Mn²⁺–O at the surface of NCM811 cathode, leading to a further increase in the stability of surface chemical states of Ni and Mn. The SEI layer of NCM811 cathode is enriched with metal fluorides and LiF, compared to the case of commercial electrolyte.

Metal-dissolution phenomena were identified by tracing the deposited metal compounds at the surface of cycled graphite anodes of full-cells under 4.45 V and −10 °C, as dissolved metal species are often transported through electrolyte and deposited at anode side. Figures 8a and 8b display XPS Ni 2p and Mn 2p spectra, respectively, for the surface of pristine and cycled graphite anodes. The spectral feature of Co 2p spectra are vague (Fig. S6). In commercial electrolyte without FEC additive, strong and broad peaks including Ni³⁺–O/Ni²⁺–O and NiF₂ at 854–856 eV (Fig. 8a–ii), and Mn⁴⁺–O/Mn²⁺–O and MnF₂ at 641.2–642.5 and 654 eV (Fig. 8b–ii), respectively, are observed. ^{7,23,24,36,39,40} These data prove the occurrence of metal-dissolution and deposition phenomena under −10 °C. The presence of deposited Ni and Mn compounds is examined with EDS elemental mapping results (Fig. 8c), which is a relative measure for the quantity of deposited metals. The presence of metal-deposits is subject to roughened surface of graphite anode, blocking of Li⁺ ion diffusion to graphite interlayers and unknown side reactions with electrolyte, impedance rise (Fig. 2a'), deleterious cycling performance (Fig. 2e). In addition to lithium metal plating, such surface degradation of cathode cause oxygen release along with electrolyte decomposition that causes an increase of gas pressure, leading to battery fire and EV fire as illustrated in Fig. 8d. With FEC additive (Figs. 8a–iii, 8b–iii), peaks of both metal-deposits are notably reduced, indicating reduced metal-dissolution. When using nonflammable electrolyte (Figs. 8a–iv and 8b–iv), the XPS features of both metal-deposits are negligible, consistent with EDS results (Fig. 8c). In nonflammable electrolyte with FEC additive, no metal deposit is shown in both XPS and (Figs. 8a–v, 8b–v) and EDS mapping results (Fig. 8c). This dictates that metal-dissolution and deposition phenomena are inhibited, due to the formation of stable SEI layers at both anode and cathode under 4.45 V and −10 °C as illustrated in Fig. 8e. Then, propagation of structural degradation from surface toward bulk is cut-off, and thereby, the bulk structure of both anode and cathode are stabilized. This is the key why the full-cell with nonflammable electrolyte with FEC additive outperforms the cell with commercial electrolyte and make the LIB safe, despite under high charge voltage and subzero-temperature.

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