  Phase diagram of Na$_x$CoO$_2$ (x $\gtrsim$ 0.71) has been reinvestigated
using electrochemically fine tuned single crystals. Both phase separation and
staging phenomena as a result of sodium multi-vacancy cluster ordering have
been found. Phase separation phenomenon is observed in the narrow ranges of
0.76 $\lesssim$ x $\lesssim$ 0.82 and 0.83 $\lesssim$ x $\lesssim$ 0.86. While
x = 0.820 shows A-type antiferromagnetic (A-AF) ordering below 22K, x = 0.833
is confirmed to have a magnetic ground state of A-AF ordering below $\sim$8K
and is only reachable through slow cooling. In addition, x = 0.859 is found to
be responsible for the highest A-AF transition temperature at about 29K.
Staging model based on ordered stacking of multi-vacancy layers is proposed to
explain the hysteretic behavior and A-AF correlation length for x $\sim$
0.82-0.86.
