Copper sulfide of the formula CuxSy, wherein x and y are integer or non-integer values, wherein (i) the copper sulfide has a sulfur 2p XPS spectrum with peaks at 162.3 eV (±1 ev), 163.8 eV (±1 ev) and 68.5 eV (±1 ev), characterised in that the peak at 168.5 eV has a lower value of counts per second (CPS) than both the peak at 162.3 eV and the peak at 163.8 eV; and (ii) the copper sulfide has a copper 2p XPS spectrum with peaks at 932.0 eV (±2 ev) and 933.6 eV (±3 eV) and characterised in that the XPS spectrum does not comprise identifiable satellite peaks at 939.8 eV and 943.1 eV (±3 eV).