Insight into Prolonged Cycling Life of 4 V All‐Solid‐State Polymer Batteries by a High‐Voltage Stable Binder

Abstract

components of high-performance SSBs is the solid-state electrolyte (SSE). Oxidebased SSEs, [2] sulfide-based SSEs, [3] halidebased SSEs, [4] polymer-based SSEs, [5] and hybrid electrolytes [6] are regarded as the most encouraging candidates for applications in SSBs. [7] Among them, polyethylene oxide (PEO) based solid polymer electrolytes (SPEs) show great promising due to its high ionic conductivity at elevated temperature, low interfacial resistance toward electrodes, and simple fabrication process. [8] More importantly, all-solid-state polymer batteries (ASSPBs) with lithium metal anode, SPE and LiFePO 4 cathode have been commercialized and used in the Bolloré Bluecar, [5] which clearly demonstrates the great capability of SPE for SSBs for EV application.However, it has been found that the state-of-the-art PEO-based SPEs developed so far delivered poor electrochemical performance when coupling with high energy density cathodes, such as lithium cobalt oxide (LiCoO 2 ), layer structure lithium nickel manganese cobalt oxide (NMC), and lithium nickel cobalt aluminum oxide (NCA). [9] This is because PEO-based SPEs have a relatively low electrochemical oxidation potential-less than 3.8 V versus Li/Li + . [10] However, these high energy density cathodes typically require charging voltages up to 4.2 V or higher to achieve a high specific capacity. At these voltages range, PEO-based SPEs will undergo electrochemical oxidation decomposition. [10,11] In order to address this serious limitation, significant research efforts Polyethylene oxide (PEO) based solid polymer electrolytes (SPEs) are incompatible with the 4 V class cathodes such as LiCoO 2 due to the limited electrochemical oxidation window of PEO. Herein, a number of binders including commonly used binders PEO, polyvinylidene fluoride (PVDF), and carboxylrich polymer (CRP) binders such as sodium alginate (Na-alginate) and sodium carboxymethyl cellulose, are studied for application in the 4 V class all-solid-state polymer batteries (ASSPBs). The results show ASSPBs with CRP binders exhibit superior cycling performance up to 1000 cycles (60% capacity retention, almost 10 times higher than those with PEO and PVDF binders). Synchrotron-based X-ray absorption spectroscopy (XAS), morphology studies and density functional theory studies indicate that, with their carboxyl groups, CRPs can strongly bind the electrode materials together, and work as coating materials to protect the cathode/SPE interface. Cyclic voltammetry studies indicate that CRP binders are more stable at high voltage compared to PEO and PVDF. The stability under high voltage and the coating property of CRP binders contribute to stable cathode/SPE interfaces as disclosed by the X-ray photoelectron spectroscopy and Co L-edge XAS results, enabling long cycling life, high performance 4 V class ASSPBs.

Understanding the capacity fade in polyacrylonitrile binder-based LiNi0.5Mn1.5O4 cells

“…Each graph is divided into three regions: region I, Mn 3+ to Mn 4+ ; region II, Ni 2+ to Ni 3+ ; and region II, Ni 3+ to Ni 4+ corresponding to the three main redox steps of LNMO. A selection of 14 mass channels were chosen, namely m/z 2, 15,16,17,18,26,27,28,32,44,52, 85 and 91, to monitor any potential volatile degradation products. Only 5 of the chosen 14 mass channels were able to detect some form of gassing.…”

“…[15] Many polymers, however, can display significant electrochemical decomposition at the high operating voltages required for LNMO cells. [16] It is therefore important to ensure that when choosing a polymer either as a binder or as a coating for LNMO, it should be oxidatively stable at higher voltages, or that its decomposition products protect from further decomposition of the electrolyte, analogous to the solid electrolyte interphase (SEI) at the anode.…”

Toward High Performance All‐Solid‐State Lithium Batteries with High‐Voltage Cathode Materials: Design Strategies for Solid Electrolytes, Cathode Interfaces, and Composite Electrodes

“…[ 182a ] Recently, Liang et al studied the effects of different types of binders (PEO, PVDF, and carboxyl‐rich polymer (CRP) binders) on the performance of high‐voltage LCO‐based ASSLBs. [ 193 ] Their results indicate that CRP binder not only offers strong binding with electrode materials and thus maintains excellent cathode structure stability but also acts as a coating material to protect the cathode–SEs interface. Furthermore, the CRP binder exhibits better high‐voltage stability compared to the PEO and PVDF binders, which also contributes to the stable cathode–SEs interface.…”

A critical discussion on the analysis of buried interfaces in Li solid-state batteries. Ex situ and in situ/operando studies

“…XPS is the most widely used method to study the chemical evolution of interfaces in SSBs. Paradoxically; almost all the reported experiments rely on the analysis of the surface of the materials obtained after the manual separation 23,24,26,[114][115][116][117] or manual etching 31,47 of the electrode/solid electrolyte interface. These operations can be easier for the Li/solid electrolyte interface, but in all cases, the procedures lack reproducibility and raise concerns about if interphases are being actually probed.…”