battery electrodes

I have been driving a Chevy VOLT for a year and a half. I have more than 26,000 miles on it, and have used 100 gallons of gasoline. That works out to more than 250 mpg. Of course, I have been charging the VOLT at home every night, and at the office during the day but my electric bills at both places are not noticeably higher. It would be nice if the electric range were a bit longer