Tailoring polymer electrolyte ionic conductivity for production of low- temperature operating quasi-all-solid-state lithium metal batteries

Abstract

The stable operation of lithium-based batteries at low temperatures is critical for applications in cold climates. However, low-temperature operations are plagued by insufficient dynamics in the bulk of the electrolyte and at electrode|electrolyte interfaces. Here, we report a quasi-solid-state polymer electrolyte with an ionic conductivity of 2.2 × 10^-4 S cm^-1 at -20 °C. The electrolyte is prepared via in situ polymerization using a 1,3,5-trioxane-based precursor. The polymer-based electrolyte enables a dual-layered solid electrolyte interphase formation on the Li metal electrode and stabilizes the LiNi_0.8Co_0.1Mn_0.1O₂-based positive electrode, thus improving interfacial charge-transfer at low temperatures. Consequently, the growth of dendrites at the lithium metal electrode is hindered, thus enabling stable Li||LiNi_0.8Co_0.1Mn_0.1O₂ coin and pouch cell operation even at -30 °C. In particular, we report a Li||LiNi_0.8Co_0.1Mn_0.1O₂ coin cell cycled at -20 °C and 20 mA g^-1 capable of retaining more than 75% (i.e., around 151 mAh g^-1) of its first discharge capacity cycle at 30 °C and same specific current.

Fig. 1. Design of polymer electrolytes for low-temperature Li metal batteries.

a Comparison of the melting points and viscosity (25 °C) of various solvents. b HOMO and LUMO energies for commonly used Li salts, solvents, TXE monomer, PEO, and POM polymers. c Schematic representation of the solid electrolyte interphase (SEI) formed on the Li metal electrode (left) and degradation processes happening in a Li||NCM811 cell (right) using a non-aqueous carbonate-based electrolyte solution. d Schematic representation of the solid electrolyte interphase (SEI) formed on the Li metal electrode (left) and inhibition of the degradation processes in a Li||NCM811 cell (right) using the polymer electrolyte reported in the present research work. The charge voltage refers to the voltage of a Li||NCM811 cell during the charging process.

Fig. 2. Electrochemical properties of the designed polymer electrolyte at various temperatures.

a Ionic conductivities as a function of temperature. b Chronoamperometry profile collected from a symmetric Li||Li cell (inset shows the EIS plots of the Li||Li coin cell before and after chronoamperometry). c Cell CEs of both electrolytes in Li||Cu coin cells. d Galvanostatic cycling of both electrolytes in symmetric Li||Li coin cells, insets: the magnification of voltage profiles during 460–500 h and 960–1000 h.

Fig. 3. Electrochemical energy storage performances of Li||NCM811 cells with polymer or liquid electrolytes cycled at various temperatures.

a Charge-discharge voltage profiles (the second cycle at each temperature, 2.8–4.5 V) of the designed polymer electrolyte in the Li||NCM811 coin cell at different temperatures and 20 mA g⁻¹. b Discharge capacities with CEs of both electrolytes in Li||NCM811 coin cells at different temperatures and 20 mA g⁻¹. c Cycling performances of both electrolytes in Li||NCM811 coin cells at −20 °C and 20 mA g⁻¹. d Charge–discharge voltage profiles of the designed polymer electrolyte in the Li||NCM811 pouch cell at −20 °C and 20 mA g⁻¹. e A Li||NCM811 pouch cell using the designed polymer electrolyte was powering an electric fan at −48.2 °C. In the above cells, NCM811 cathode with a mass loading of 2.5 mg cm⁻² and anode of 50 μm Li foil was used. The mass of the specific capacity and specific current refers to the mass of the active material in the positive electrode.

Fig. 4. Ex situ postmortem physicochemical characterizations of Li metal electrodes cycled in liquid and polymer electrolytes.

a–d SEM images illustrating morphologies of the LMAs cycled in Li||Li coin cells with different electrolytes: surface morphologies (a) and cross-section views (b) using the designed polymer electrolyte, and surface morphologies (c) and cross-section views (d) using the liquid electrolyte. (insets a and c show the optical photographs of cycled LMAs). e, f XPS depth profiles of C 1s, F 1s, B 1s, and Li 1s in LMAs cycled in Li||Li coin cells with the designed polymer electrolyte (e), and the liquid electrolyte (f). g Relative compositions of Li-containing species. The characterized LMAs were collected from Li||Li coin cells after 100 cycles at −20 °C and 0.2 mA cm⁻².

Fig. 5. Physicochemical characterizations of the Li metal electrode SEI via ex situ postmortem measurements.

a, b Depth profiles of the Li SEIs in Li||Li coin cells with the designed polymer electrolyte (a), and the liquid electrolyte (b). c, d 3D renders of the Li SEIs in Li||Li coin cells with the polymer electrolyte (c), and the liquid electrolyte (d). The characterized LMAs were collected from Li||Li coin cells after 100 cycles at −20 °C and 0.2 mA cm⁻². e, f Cryo-TEM images of deposited Li in the cell with the polymer electrolyte at different scales. g Corresponding FFT pattern of the inner SEI: green circle: LiF; red circle: Li; yellow circle: Li₂O; blue circle: Li₂CO₃. h Schematic of the observed dual-layered SEI on the deposited Li. The Li sample for cryo-TEM was prepared by depositing Li on a TEM grid, at −20 °C and 0.2 mA cm⁻² cm (to a capacity of 0.02 mAh cm⁻²), with the designed polymer electrolyte.

Fig. 6. Ex situ postmortem physicochemical characterizations of NCM811-based electrodes cycled in liquid and polymer electrolytes.

a, b SEM images of NCM811 particles using the designed polymer electrolyte (a), and the liquid electrolyte (b). c, d TEM of NCM811 particles using the polymer electrolyte (c), and the liquid electrolyte (d). e–h High-resolution TEM images and corresponding FFT for NCM811 particles cycled with the polymer electrolyte (e, g), and the liquid electrolyte (f, h). i–l XPS spectra of C 1s (i), F 1s (j), B 1s (k), and Ni 2p (l) for NCM811 cathodes using both electrolytes. The NCM811-based electrodes were collected from Li||NCM811 coin cells after 100 cycles at −20 °C and 40 mA g⁻¹. The specific current refers to the mass of the active material in the positive electrode.

Fig. 7. Electrochemical energy storage performances of Li||NCM811 cells with polymer or liquid electrolytes cycled at 30 °C.

a Rate performances of both electrolytes, and b charge–discharge voltage profiles (second cycle) of the Li||NCM811 coin cell with the designed polymer electrolyte at different applied currents. In these coin cells, NCM811 cathode with a mass loading of 2.5 mg cm⁻² and anode of 50 μm Li foil was used. c Cycling performance of Li||NCM811 coin cells with both electrolytes under practical conditions. The first two formation cycles were carried out at 20 mA g⁻¹, and the long-term cycling was set at 40 mA g⁻¹. d Charge–discharge voltage profiles of the Li||NCM811 pouch cell with the polymer electrolyte. In the Li||NCM811 coin and pouch cell under practical conditions, high-loading NCM811 cathodes (~2.5 mAh cm⁻²) and 50 μm Li foil anodes were used, where the N/P ratio was ~3.86 and the E/C ratio was ~5 g (Ah)⁻¹. The mass of the specific capacity and specific current refers to the mass of the active material in the positive electrode.