The electrochemical performances of the spinels Li[Ni0.5M1.5]O-4, Li[CrM]O-4, and Li[MnM]O-4 with M=Mn(IV) vs Ti(IV) as cathodes for Li-ion batteries are compared. With M=Mn(IV), reversible access to the valence states Ni(IV) to Ni(II) and Cr(IV) to Cr(III) is possible at a voltage V approximate to 4.7 and 4.85 V (vs Li+/Li), respectively. The solid electrolyte interface (SEI) layer formed with M=Mn(IV) at voltages V>4.3 V are Li-permeable. The disproportionation reaction 3Cr(IV)=2Cr(III)+Cr(VI) contributes to the loss of capacity in Li[CrMn]O-4. With M=Ti(IV), reversible charge/discharge curves were not obtained between 3.5 and 4.9 V (vs Li+/Li). The cathode Fermi energy E-FC is lowered by about 0.2 eV on charging from M=Mn(IV) to M=Ti(IV), but this lowering is not sufficient to cause O-2 evolution from the spinel on charge with M=Ti(IV). Whereas Li-permeable SEI layers are formed with M=Mn(IV), we conclude that the SEI layers formed with M=Ti(IV) are Li-blocking. The SEI layer may be a Li-poor, Ti-rich phase formed at the surface of the cathode particle during charge rather than an SEI layer formed by a reaction with the electrolyte. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3392368] All rights reserved.