How can a Li-Ion battery maintain decent characteristics for nine years?
Batteries made by Yardley Lithion

Well discussed here:

Life verification of large capacity Yardney Li-Ion cells and batteries in
support of NASA missions

One version 60% capacity after 3000 cycles deep discharge 

Another - 25,000 cycles, 40% depth of discharge 35 min charge/discharge periods (LEO satellite simulation). Max Voltage = 3.8V/cell!

Next Generation and Domestically Produced Active Materials for Lithium-Ion Cells 
for Military and Aerospace Applications



Adding material from comments that increases usefulness of answer

PJC in Jan 2013 noted that majopr points from the paper cited above included 


"the capacity fade at lower temperature is significantly decreased" (-20 degrees C).  


and


" ... also to keep the rate and depth of discharge low ("battery could not support rate of discharge >C/2 at -20C when &lt;75% charged after 1010 sols of operation"), and for long term storage to keep the batteries at 50-70% charge, not fully charged."
He also noted - The anode composition and surface coating may also be different from consumer batteries.