Boron-Doped Methylated Amorphous Silicon for Negative Electrodes in Li-Ion Batteries

Abstract

In spite of its outstanding capacity for alloying with lithium, silicon cannot be practically used as a negative electrode for Li-ion batteries: its large volume expansion upon lithiation leads to a poor capacity retention [1]. Promising results have been obtained by incorporating methyl groups in amorphous silicon (methylated amorphous silicon). This material exhibits an improved stability upon electrochemical cycling while keeping a capacity close to that of pure silicon [2]. However, the conductivity of methylated amorphous silicon may be a strong limitation, especially at high methyl content: for example, 10% methylated amorphous silicon is 10000 more resistive than pure amorphous silicon.Doping is a well-known method to enhance the electronic conductivity of semiconductors, even if the dopant activity is lower in amorphous semiconductors than in crystalline ones. 2% boron doping increases the conductivity of 10% methylated amorphous silicon by five orders of magnitude compared to the undoped material.Boron doped methylated silicon thin films (100nm thick) with various methyl content were cycled in the range 0.025V – 1V at C/2 rate (electrolyte: LP30 with 5%FEC). 10% methylated amorphous silicon with 2% boron doping exhibits a capacity retention of 70% after 500 cycles of full lithiation/delithiation, an improved performance as compared to the undoped material (see Figure 1a). Interestingly, boron doping allows for using higher methyl content without demanding pre-conditioning procedures for the electrochemical cycling of the material. The stability upon cycling is found to be further increased for 15% and 20% methylated electrodes, with a capacity retention exceeding 80% over 1000 cycles of full lithiation/delithiation (Figure 1b). This figure comes at the expense of a decreased total capacity (which remains 3 to 4 times larger than that of the current carbon electrodes). The SEI evolution and structural changes are currently investigated using operando ATR FTIR and ex-situ Raman spectroscopies, in order to rationalize the factors limiting the Coulombic efficiency to 99.7%.