Have you considered lithium ion batteries or flywheels?
Although to date much less frequently utilized as sources for backup power, these three alternatives are proven viable solutions and worthy of consideration when it's time to replace VRLA batteries.
The most commonly utilized backup power source for uninterruptible power supplies has historically been lead acid batteries, either VRLA (valve regulated lead acid) or VLA (vented lead acid, also referred to as wet cell or flooded). However, their inherent challenges of relatively short life span (4-5 years), extensive maintenance requirements (semi-annually), and excessive weight & size have in part triggered the development of alternative energy sources, most notably, various chemistries of lithium ion solutions. Mitsubishi Electric has studied the landscape of the available emerging chemistries and confidently offers three of the best in its product offering, each possessing its own unique benefits. All 3 solutions offer many advantages over VRLA.
Flywheels are certainly not a new technology and like anything else, they have advantages and disadvantages. On the plus side, they have a small footprint, require very little maintenance, and are an economic alternative compared to other solutions. On the minus side, they have a relatively short runtime, and therefore limited applications. However, they represent an attractive solution for industries desiring the absence of potentially hazardous materials or are comfortable with runtimes of 60 seconds or less.
Flywheels are frequently the energy storage device of choice for hospitals due to their absence of (spillable) lead acid.
Upgrading to pure lead batteries remains another option to employing VRLA batteries. In some cases, a sub 5 minute runtime is all that is needed, and a pure lead solution can accomplish this with less cabinets and a smaller footprint. Pure lead batteries deliver other desirable characteristics, such as prolonged lifespan, longer warranties, and higher tolerance for increased temperatures. Of course, these more appealing features come with a higher price tag, so you will need to assess what your priorities are ahead of making a selection.