In a first aspect, the invention provides a process for preparing silicon-containing composite particles, the process comprising the steps of: (a) providing a plurality of porous particles comprising micropores and/or mesopores, wherein: (i) the D50 particle diameter of the porous particles is in the range from 0.5 to 200 µm; (ii) the total pore volume of micropores and mesopores as measured by gas adsorption is in the range from 0.4 to 2.2 cm3/g; (iii) the PD50 pore diameter as measured by gas adsorption is no more than 30 nm; (b) combining a charge of the porous particles with a charge of a silicon-containing precursor in a batch pressure reactor, wherein the charge of the porous particles has a volume of at least 20 cm3 per litre of reactor volume (cm3/LRV), preferably at least 200 cm3 per litre of reactor volume (cm3/LRV), and wherein the charge of the silicon-containing precursor comprises at least 2 g of silicon per litre of reactor volume (g/LRV); and (c) heating the reactor to a temperature effective to cause deposition of silicon in the pores of the porous particles, thereby providing the silicon-containing composite particles.