  We present a structural and magnetic study on two batches of polycrystalline
LiNi$_{0.8}$Mn$_{0.1}$Co$_{0.1}$O$_2$ (commonly known as Li NMC 811), a Ni-rich
Li ion battery cathode material, using elemental analysis, X-ray and neutron
diffraction, magnetometry, and polarised neutron scattering measurements. We
find that the samples, labelled S1 and S2, have the composition
Li$_{1-x}$Ni$_{0.9+x-y}$Mn$_y$Co$_{0.1}$O$_2$, with $x = 0.025(2)$, $y =
0.120(2)$ for S1 and $x = 0.002(2)$, $y = 0.094(2)$ for S2, corresponding to
different concentrations of magnetic ions and excess Ni$^{2+}$ in the
Li$^+$layers. Both samples show a peak in the zero-field cooled (ZFC) dc
susceptibility at 8.0(2) K but the temperature at which the ZFC and FC
(field-cooled) curves deviate is substantially different: 64(2) K for S1 and
122(2) K for S2. Ac susceptibility measurements show that the transition for S1
shifts with frequency whereas no such shift is observed for S2 within the
resolution of our measurements. Our results demonstrate the sample dependence
of magnetic properties in Li NMC 811, consistent with previous reports on the
parent material LiNiO$_2$. We further establish that a combination of
experimental techniques are necessary to accurately determine the chemical
composition of next generation battery materials with multiple cations.
