While searching for transformerless inverters in the IEEE site, most of them seem to be about grid connected Photovoltaic inverters. Removing transformers will increase the efficiency at the cost of galvanic non-isolation between the ac and dc parts which might cause dangerous dc injection currents. Is this why battery inverters do not have transformerless topologies?
Grid-tied PV systems use a high DC bus voltage — higher than the peak AC voltage — which means that it is simple to use a single set of switching elements to turn this into the required AC output.
Battery-based systems generally use a much lower DC bus voltage. In order to make a transformerless inverter, you'd need to first boost the bus voltage to the higher value using one set of switching elements (DC-DC converter), and then have another set of switching elements to do the waveform generation. It's much simpler, and generally just as efficient overall, to do the waveform generation at the battery bus voltage and use a transformer to boost it to the line level.
In addition, in a UPS application, you can use that same transformer and switching elements to charge the battery, which reduces the overall system cost.