No Significant Effect of Phosphorous Doping on the Electrochemical Performance of Silicon-Carbon Composite Anodes for Li-ion Batteries

Abstract

SUMMARY Using an industry-relevant method of production we have demonstrated production of doped silicon nanoparticles for Li-ion batteries. The particles have been characterized and tested in Li-ion battery half-cells for demonstration of the performance as Li ion storage material in the anode. In this presentation we will show the results regarding the characterization of the particles as well as the performance results of the material in Li-ion half cells. BACKGROUND The use of silicon as part of the anode in Li-ion batteries enhances the potential storage capacity greatly. While the graphite anode used in standard Li-ion batteries has a theoretical capacity of 372 mAh/g, the theoretical capacity for Si is 3,572 mAh/g [1]. However, to be able to absorb the sufficient amount of Li ions reversibly, the silicon material needs to be nanostructured [2]. EXPERIMENTAL IFE has earlier demonstrated production of silicon particles prepared using a silane-based free-space reactor, which allows control of particle size distribution and crystallinity and a high production capacity [3]. In this work, the setup was modified to allow for introduction of phosphine gas in the production process. The intention was to introduce phosphorous dopant atoms to modify the electrical conductivity of the silicon material. The current particles were produced with a silane flow rate of 5 slm, and 5 slm of 3% phosphine diluted in He. Thus, the P concentration in the process gases was 3% with respect to Si. The production rate at 100% yield would be about 400 g/h, while the product recovered from the process typically was between 100 and 200 g/h. The processing temperature was varied from 475 °C to 600 °C to produce particles with different properties. All electrochemical tests were performed in a crimped 2032 coin cell with lithium metal used as a counter electrode, with a polymer separator (Celgard 3401) and 1 M LiPF6 in 1:1 EC/DMC electrolyte (LP30, BASF). For some experiments, 10 wt% fluoroethylene carbonate (FEC) was used as electrolyte additive. The cells were cycled at 25 °C between 0.05 and 1.0 V with a constant C-rate of C/10 (after initial 3 cycles at C/20) using an Arbin Battery cycler (Arbin Instruments). RESULTS AND CONCLUSIONS The phosphorus content of the particles was by inductively coupled plasma mass spectrometry (ICP-MS) and energy-dispersive x-ray spectroscopy (EDS) measured to be in the range of 1.7-1.8% with respect to silicon. Thus, it seems that the utilization of the phosphine is not complete, or phosphorous binds to materials which are not recovered from the production process. The particle size distribution was centered around 600 nm as measured by laser scattering in a Malvern Mastersizer 2000. SEM and TEM revealed that the particles consisted of aggregates of spherical silicon particles with a primary particle size of 50-400 nm. X-ray diffraction showed that we could produce amorphous or crystalline particles depending on the temperature of the process. Cycling of the particles in lithium battery half cells showed a life time of about 300 cycles for doped and undoped particles. Some measurements indicate a better conductivity of the doped material but the results were not conclusive. The cells were cycled with a limited capacity regime where the capacity was set to 1000 mAh/g(Si) and thus the end-voltage could vary. The most pronounced difference in the cycling performance between doped and undoped silicon particles was a shift in the increase in end voltage for the doped particles: While end-voltage increased from 0.4 V to 0.6 V over the first 25 cycles, this development is stretched out over the first 50 cycles or more for doped particles. See the attached figure for further details.