As already mentioned, the new USB specifications do allow for higher than 5V charging voltage, if the device requests it.
Under the "USB Power Delivery" (PD) extension (USB PD rev. 3.0, ver 1.0a now), up to as much as 5A at 5V, 9V, 15V, or 20V can be provided to the attached device, with a maximum of 100W sourced. There are many rules the "requesting device" must follow for these cases.
This would typically be for notebook computers or other devices that have higher cell counts and/or operating voltages. The cables must also be designed and specified to handle these conditions.