A Long-Life 3-Stack 12 V Dry Polymer Battery Using LiNi1/3Mn1/3Co1/3O2 and Graphite

Introduction

Safety is more important than high energy in stationary batteries. In addition, material choices based on the low-cost and easy production of larger batteries is another critical issue for developing large energy storage systems comparable to pumped hydro systems. A solvent-free solid polymer electrolyte (SPE) has the advantages of being safe, inexpensive and barrier-free for large battery production. However, polyether-based SPEs are believed to oxidize at >4 V. Carbon-based anodes are also thought to be incompatible with polyether-based SPEs. Therefore, such SPEs are now in categorized as traditional electrolytes. However, we demonstrated (i) the compatibility of 4 V-5 V cathodes, such as LiCoO₂¹⁾ and LiNi_0.5Mn_1.5O₄²⁾, by modifying the cathode and polyether-based SPE interface and (ii) sufficient graphite reversibility with the SPE using suitably prepared electrodes. Recently, we drastically improved the cycle operation (>1,500 cycles) of LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)³⁾ and graphite using an optimized lithium salt and by introducing carboxymethylcellulose (CMC) at the interface. These materials are already mass produced; therefore, the active material cost will be equivalent to conventional lithium-ion batteries. There are no flammable vapors in the electrolyte (a pure polymer rather than a gel); therefore, an intrinsic safety improvement is expected for abused cells. In addition, a simple, multi-cell design using one external package is another advantage of all solid state batteries. Based on these advantages and renovations, we readopted such traditional SPE systems for stationary batteries. Here, we further improved the cycle performance by adding a new lithium salt and demonstrated the long term operation of a [Graphite | SPE | NMC] × 3 stack 12 V single-package multi-cell.

Experimental

The electrode materials were mixed with CMC, styrene-butadiene rubber (SBR) and conductive additives in a solvent without any SPEs before coating on the current collector. Subsequently, a polyether-based SPE (M=1.5 M, Daiso Co., Ltd.) was overcoated on the electrode using acetonitrile (AN). LiBF₄ was used in the overcoat solution for NMC. In contrast, LiTFSI was used in the overcoat solution for graphite and the SPE sheet between the two electrodes. A lithium-ion cell composed of NMC, SPE, and graphite was fabricated using an Al-laminate-type cell. For the multi-cell, three cells (effective surface area 10 ×10 cm) were stacked in series in one package. All cells operated at 50°C.

Results and Discussion

We added 5 % LiBOB to the NMC overcoat solution. This LiBOB additive remarkably improved the reversibility, as shown in Fig. 1. The LiBOB additive was reported to suppress Al corrosion and/or Mn dissolution. While the detailed mechanism for these improvements is currently being studied, the high retention capacity obtained (70 % at 3,000 cycles) is practical for stationary use. Furthermore, over 1,000 operating cycles were successfully performed using a 10×10 cm three stack 12 V multi-cell, as shown in Fig. 2. The stacked multi-cell was vacuum sealed so no additional pressure was required during operations, which is significant progress for a large format battery design.

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