Room-Temperature NMC811/Silicon Solid-State Battery Enabled by Hydroborate Electrolyte

To realize the full potential of solid-state batteries, the use of high-capacity negative electrodes such as lithium metal or silicon is needed. Silicon represents a safer alternative to lithium metal due to its higher lithiation potential, reducing the risk of dendrite formation. Silicon is challenging to operate at industry-relevant areal capacities with high capacity retention because of its volume change upon de-/lithiation. Furthermore, the insufficient reductive stability of commonly used Li 6 PS 5 Cl electrolyte limits the electrode design to pure silicon electrodes with a 2D interface to the electrolyte, because the electrode-electrolyte surface area needs to be minimized. These 2D silicon electrodes typically require unrealistically high stack pressures of 50 MPa for reliable operation. 1-3 Hydroborate solid electrolytes present an attractive alternative since they combine many attractive characteristics such as high ionic conductivity, high thermal stability, soft mechanical properties, and non-toxicity. We recently showed that hydroborates are compatible with high-voltage LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) positive electrodes. 4 In this work, we demonstrate that the hydroborate electrolyte Li 3 (CB 11 H 12 ) 2 (CB 9 H 10 ) also offers excellent kinetic stability against reduction in contact with lithiated silicon. 5 This property enables stable cycling of 3D silicon composite electrodes, even when using nano-sized silicon particles and conductive carbon additives. The lithiated 3D silicon composite electrodes feature a low interface resistance growth rate of ~ 0.3 Ω cm 2 h −0.5 at room temperature, in contrast to the rapid interface resistance growth rate of ~ 10 Ω cm 2 h −0.5 observed for the Li 6 PS 5 Cl electrolyte. 1, 2 An excellent initial coulombic efficiency of 96% is reached in half-cells at 60 °C, at a high reversible capacity of 3100 mAh g Si −1 , corresponding to an areal capacity of 8.9 mAh cm −2 . Owing to its high room-temperature conductivity of 1.5 mS cm −1 and wide electrochemical stability window, Li 3 (CB 11 H 12 ) 2 (CB 9 H 10 ) further enables successful integration of the 3D silicon composite electrode with the previously reported composite NMC811 electrode in a zero-lithium-excess full cell with an areal capacity of 3 mAh cm −2 . This NMC811/silicon full cell achieves a remarkable capacity retention of 66% after 100 cycles and respectable rate capability of 2 mAh cm −2 at 1 mA cm −2 at room temperature and under 50 MPa stack pressure. Importantly, a capacity of 2.8 mAh cm −2 is retained under moderate stack pressure of 8 MPa, highlighting the benefits of the 3D composite electrode design.